diff --git a/R/RcppExports.R b/R/RcppExports.R
index b5c7b6d0..cfd5e074 100644
--- a/R/RcppExports.R
+++ b/R/RcppExports.R
@@ -53,7 +53,7 @@ parse_ref <- function(ref) {
     .Call('readxl_parse_ref', PACKAGE = 'readxl', ref)
 }
 
-xlsx_col_types <- function(path, sheet = 0L, na = "", nskip = 0L, n = 100L) {
+xlsx_col_types <- function(path, sheet = 0L, na = character(), nskip = 0L, n = 100L) {
     .Call('readxl_xlsx_col_types', PACKAGE = 'readxl', path, sheet, na, nskip, n)
 }
 
diff --git a/src/CellType.h b/src/CellType.h
index 31f811d4..ef93b415 100644
--- a/src/CellType.h
+++ b/src/CellType.h
@@ -3,6 +3,7 @@
 
 #include <Rcpp.h>
 #include <libxls/xls.h>
+#include "StringSet.h"
 
 enum CellType {
   CELL_BLANK,
@@ -48,21 +49,21 @@ inline std::string cellTypeDesc(CellType type) {
 
 inline CellType cellType(xls::st_cell::st_cell_data cell, xls::st_xf* styles,
                          const std::set<int>& customDateFormats,
-                         std::string na = "") {
+                         const StringSet &na = "") {
   // Find codes in [MS-XLS] S2.3.2 (p175).
   // See xls_addCell for those used for cells
   switch(cell.id) {
   case 253: // LabelSst
   case 516: // Label
-    return (na.compare((char*) cell.str) == 0) ? CELL_BLANK : CELL_TEXT;
+    return na.contains((char*) cell.str) ? CELL_BLANK : CELL_TEXT;
     break;
 
   case 6:    // formula
   case 1030: // formula (Apple Numbers Bug)
     if (cell.l == 0) {
-      return CELL_NUMERIC;
+      return na.contains(cell.d) ? CELL_BLANK : CELL_NUMERIC;
     } else {
-      if (na.compare((char*) cell.str) == 0) {
+      if (na.contains((char*) cell.str)) {
         return CELL_BLANK;
       } else {
         return CELL_TEXT;
@@ -74,6 +75,9 @@ inline CellType cellType(xls::st_cell::st_cell_data cell, xls::st_xf* styles,
   case 515: // Number
   case 638: // Rk
     {
+      if (na.contains(cell.d))
+        return CELL_BLANK;
+
       if (styles == NULL)
         return CELL_NUMERIC;
 
diff --git a/src/RcppExports.cpp b/src/RcppExports.cpp
index 99d3b726..029c0166 100644
--- a/src/RcppExports.cpp
+++ b/src/RcppExports.cpp
@@ -64,13 +64,13 @@ BEGIN_RCPP
 END_RCPP
 }
 // xls_col_types
-CharacterVector xls_col_types(std::string path, std::string na, int sheet, int nskip, int n, bool has_col_names);
+CharacterVector xls_col_types(std::string path, std::vector<std::string> na, int sheet, int nskip, int n, bool has_col_names);
 RcppExport SEXP readxl_xls_col_types(SEXP pathSEXP, SEXP naSEXP, SEXP sheetSEXP, SEXP nskipSEXP, SEXP nSEXP, SEXP has_col_namesSEXP) {
 BEGIN_RCPP
     Rcpp::RObject __result;
     Rcpp::RNGScope __rngScope;
     Rcpp::traits::input_parameter< std::string >::type path(pathSEXP);
-    Rcpp::traits::input_parameter< std::string >::type na(naSEXP);
+    Rcpp::traits::input_parameter< std::vector<std::string> >::type na(naSEXP);
     Rcpp::traits::input_parameter< int >::type sheet(sheetSEXP);
     Rcpp::traits::input_parameter< int >::type nskip(nskipSEXP);
     Rcpp::traits::input_parameter< int >::type n(nSEXP);
@@ -80,7 +80,7 @@ BEGIN_RCPP
 END_RCPP
 }
 // xls_cols
-List xls_cols(std::string path, int i, CharacterVector col_names, CharacterVector col_types, std::string na, int nskip);
+List xls_cols(std::string path, int i, CharacterVector col_names, CharacterVector col_types, std::vector<std::string> na, int nskip);
 RcppExport SEXP readxl_xls_cols(SEXP pathSEXP, SEXP iSEXP, SEXP col_namesSEXP, SEXP col_typesSEXP, SEXP naSEXP, SEXP nskipSEXP) {
 BEGIN_RCPP
     Rcpp::RObject __result;
@@ -89,7 +89,7 @@ BEGIN_RCPP
     Rcpp::traits::input_parameter< int >::type i(iSEXP);
     Rcpp::traits::input_parameter< CharacterVector >::type col_names(col_namesSEXP);
     Rcpp::traits::input_parameter< CharacterVector >::type col_types(col_typesSEXP);
-    Rcpp::traits::input_parameter< std::string >::type na(naSEXP);
+    Rcpp::traits::input_parameter< std::vector<std::string> >::type na(naSEXP);
     Rcpp::traits::input_parameter< int >::type nskip(nskipSEXP);
     __result = Rcpp::wrap(xls_cols(path, i, col_names, col_types, na, nskip));
     return __result;
@@ -163,14 +163,14 @@ BEGIN_RCPP
 END_RCPP
 }
 // xlsx_col_types
-CharacterVector xlsx_col_types(std::string path, int sheet, std::string na, int nskip, int n);
+CharacterVector xlsx_col_types(std::string path, int sheet, CharacterVector na, int nskip, int n);
 RcppExport SEXP readxl_xlsx_col_types(SEXP pathSEXP, SEXP sheetSEXP, SEXP naSEXP, SEXP nskipSEXP, SEXP nSEXP) {
 BEGIN_RCPP
     Rcpp::RObject __result;
     Rcpp::RNGScope __rngScope;
     Rcpp::traits::input_parameter< std::string >::type path(pathSEXP);
     Rcpp::traits::input_parameter< int >::type sheet(sheetSEXP);
-    Rcpp::traits::input_parameter< std::string >::type na(naSEXP);
+    Rcpp::traits::input_parameter< CharacterVector >::type na(naSEXP);
     Rcpp::traits::input_parameter< int >::type nskip(nskipSEXP);
     Rcpp::traits::input_parameter< int >::type n(nSEXP);
     __result = Rcpp::wrap(xlsx_col_types(path, sheet, na, nskip, n));
@@ -191,7 +191,7 @@ BEGIN_RCPP
 END_RCPP
 }
 // read_xlsx_
-List read_xlsx_(std::string path, int sheet, RObject col_names, RObject col_types, std::string na, int nskip);
+List read_xlsx_(std::string path, int sheet, RObject col_names, RObject col_types, std::vector<std::string> na, int nskip);
 RcppExport SEXP readxl_read_xlsx_(SEXP pathSEXP, SEXP sheetSEXP, SEXP col_namesSEXP, SEXP col_typesSEXP, SEXP naSEXP, SEXP nskipSEXP) {
 BEGIN_RCPP
     Rcpp::RObject __result;
@@ -200,7 +200,7 @@ BEGIN_RCPP
     Rcpp::traits::input_parameter< int >::type sheet(sheetSEXP);
     Rcpp::traits::input_parameter< RObject >::type col_names(col_namesSEXP);
     Rcpp::traits::input_parameter< RObject >::type col_types(col_typesSEXP);
-    Rcpp::traits::input_parameter< std::string >::type na(naSEXP);
+    Rcpp::traits::input_parameter< std::vector<std::string> >::type na(naSEXP);
     Rcpp::traits::input_parameter< int >::type nskip(nskipSEXP);
     __result = Rcpp::wrap(read_xlsx_(path, sheet, col_names, col_types, na, nskip));
     return __result;
diff --git a/src/StringSet.h b/src/StringSet.h
new file mode 100644
index 00000000..329ebf25
--- /dev/null
+++ b/src/StringSet.h
@@ -0,0 +1,31 @@
+#ifndef READXL_STRINGSET_
+#define READXL_STRINGSET_
+
+#include <Rcpp.h>
+
+class StringSet
+{
+  std::set<std::string> set_;
+public:
+  StringSet(const char *s = "") {
+    if (strlen(s) > 0)
+      set_.insert(s);
+  }
+  StringSet(const std::vector<std::string> &s) {
+    for (std::vector<std::string>::const_iterator i = s.begin(); i != s.end(); ++i)
+      set_.insert(*i);
+  }
+  StringSet(const Rcpp::CharacterVector &s) {
+    for (Rcpp::CharacterVector::const_iterator i = s.begin(); i != s.end(); ++i)
+      set_.insert(Rcpp::as<std::string>(*i));
+  }
+  bool contains(const std::string &s) const {
+    return set_.find(s) != set_.end();
+  }
+  bool contains(const double d) const {
+    std::ostringstream str; str << d;
+    return contains(str.str());
+  }
+};
+
+#endif
diff --git a/src/XlsWorkSheet.cpp b/src/XlsWorkSheet.cpp
index f4d57565..bb5fa337 100644
--- a/src/XlsWorkSheet.cpp
+++ b/src/XlsWorkSheet.cpp
@@ -13,7 +13,7 @@ CharacterVector xls_col_names(std::string path, int i = 0, int nskip = 0) {
 }
 
 // [[Rcpp::export]]
-CharacterVector xls_col_types(std::string path, std::string na, int sheet = 0,
+CharacterVector xls_col_types(std::string path, std::vector<std::string> na, int sheet = 0,
                               int nskip = 0, int n = 100, bool has_col_names = false) {
   XlsWorkBook wb = XlsWorkBook(path);
   std::vector<CellType> types = wb.sheet(sheet).colTypes(na, nskip + has_col_names, n);
@@ -37,7 +37,7 @@ CharacterVector xls_col_types(std::string path, std::string na, int sheet = 0,
 
 // [[Rcpp::export]]
 List xls_cols(std::string path, int i, CharacterVector col_names,
-              CharacterVector col_types, std::string na, int nskip = 0) {
+              CharacterVector col_types, std::vector<std::string> na, int nskip = 0) {
   XlsWorkBook wb = XlsWorkBook(path);
   XlsWorkSheet sheet = wb.sheet(i);
 
diff --git a/src/XlsWorkSheet.h b/src/XlsWorkSheet.h
index 72721264..6fee5165 100644
--- a/src/XlsWorkSheet.h
+++ b/src/XlsWorkSheet.h
@@ -65,7 +65,7 @@ class XlsWorkSheet {
     return out;
   }
 
-  std::vector<CellType> colTypes(std::string na, int nskip = 0, int n_max = 100) {
+  std::vector<CellType> colTypes(const StringSet &na, int nskip = 0, int n_max = 100) {
     std::vector<CellType> types(ncol_);
 
     for (int i = nskip; i < nrow_ && i < n_max; ++i) {
@@ -88,7 +88,7 @@ class XlsWorkSheet {
   }
 
   Rcpp::List readCols(Rcpp::CharacterVector names, std::vector<CellType> types,
-                      std::string na, int nskip = 0) {
+                      const StringSet &na, int nskip = 0) {
     if ((int) names.size() != ncol_ || (int) types.size() != ncol_)
       Rcpp::stop("Need one name and type for each column");
 
@@ -154,7 +154,7 @@ class XlsWorkSheet {
             SET_STRING_ELT(col, i, NA_STRING);
           } else {
             std::string stdString((char*) cell.str);
-            Rcpp::RObject rString = stdString == na ? NA_STRING : Rf_mkCharCE(stdString.c_str(), CE_UTF8);
+            Rcpp::RObject rString = na.contains(stdString) ? NA_STRING : Rf_mkCharCE(stdString.c_str(), CE_UTF8);
             SET_STRING_ELT(col, i, rString);
           }
           break;
diff --git a/src/XlsxCell.h b/src/XlsxCell.h
index 21314077..93719c46 100644
--- a/src/XlsxCell.h
+++ b/src/XlsxCell.h
@@ -107,31 +107,31 @@ class XlsxCell {
     return stringTable.at(id);
   }
 
-  double asDouble(const std::string& na) {
+  double asDouble(const StringSet& na) {
     rapidxml::xml_node<>* v = cell_->first_node("v");
-    if (v == NULL || na.compare(v->value()) == 0)
+    if (v == NULL || na.contains(v->value()))
       return NA_REAL;
 
     return (v == NULL) ? 0 : atof(v->value());
   }
 
-  double asDate(const std::string& na, int offset) {
+  double asDate(const StringSet& na, int offset) {
     rapidxml::xml_node<>* v = cell_->first_node("v");
-    if (v == NULL || na.compare(v->value()) == 0)
+    if (v == NULL || na.contains(v->value()))
       return NA_REAL;
 
     double value = atof(v->value());
     return (v == NULL) ? 0 : (value - offset) * 86400;
   }
 
-  Rcpp::RObject asCharSxp(const std::string& na,
+  Rcpp::RObject asCharSxp(const StringSet& na,
                           const std::vector<std::string>& stringTable) {
 
     // Is it an inline string?  // 18.3.1.53 is (Rich Text Inline) [p1649]
     rapidxml::xml_node<>* is = cell_->first_node("is");
     if (is != NULL) {
       std::string value;
-      if (!parseString(is, &value) || na.compare(value) == 0) {
+      if (!parseString(is, &value) || na.contains(value)) {
         return NA_STRING;
       } else {
         return Rf_mkCharCE(value.c_str(), CE_UTF8);
@@ -148,7 +148,7 @@ class XlsxCell {
     if (t != NULL && strncmp(t->value(), "s", t->value_size()) == 0) {
       return stringFromTable(v->value(), na, stringTable);
     } else {
-      if (na.compare(v->value()) == 0) {
+      if (na.contains(v->value())) {
         return NA_STRING;
       } else {
         return Rf_mkCharCE(v->value(), CE_UTF8);
@@ -156,7 +156,7 @@ class XlsxCell {
     }
   }
 
-  CellType type(const std::string& na,
+  CellType type(const StringSet& na,
                 const std::vector<std::string>& stringTable,
                 const std::set<int>& dateStyles) {
     rapidxml::xml_attribute<>* t = cell_->first_attribute("t");
@@ -182,13 +182,13 @@ class XlsxCell {
 
       int id = atoi(v->value());
       const std::string& string = stringTable.at(id);
-      return (string == na) ? CELL_BLANK : CELL_TEXT;
+      return na.contains(string) ? CELL_BLANK : CELL_TEXT;
     } else if (strncmp(t->value(), "str", 5) == 0) { // formula
       rapidxml::xml_node<>* v = cell_->first_node("v");
       if (v == NULL)
         return CELL_BLANK;
 
-      return (na.compare(v->value()) == 0) ? CELL_BLANK : CELL_TEXT;
+      return na.contains(v->value()) ? CELL_BLANK : CELL_TEXT;
     } else if  (strncmp(t->value(), "inlineStr", 9) == 0) { // formula
       return CELL_TEXT;
     } else {
@@ -203,7 +203,7 @@ class XlsxCell {
 private:
 
 
-  Rcpp::RObject stringFromTable(const char* val, const std::string& na,
+  Rcpp::RObject stringFromTable(const char* val, const StringSet& na,
                                 const std::vector<std::string>& stringTable) {
     int id = atoi(val);
     if (id < 0 || id >= (int) stringTable.size()) {
@@ -212,7 +212,7 @@ class XlsxCell {
     }
 
     const std::string& string = stringTable.at(id);
-    return (string == na) ? NA_STRING : Rf_mkCharCE(string.c_str(), CE_UTF8);
+    return na.contains(string) ? NA_STRING : Rf_mkCharCE(string.c_str(), CE_UTF8);
   }
 
 };
diff --git a/src/XlsxWorkSheet.cpp b/src/XlsxWorkSheet.cpp
index a5128742..67796a1b 100644
--- a/src/XlsxWorkSheet.cpp
+++ b/src/XlsxWorkSheet.cpp
@@ -22,7 +22,7 @@ IntegerVector parse_ref(std::string ref) {
 
 // [[Rcpp::export]]
 CharacterVector xlsx_col_types(std::string path, int sheet = 0,
-                               std::string na = "", int nskip = 0,
+                               CharacterVector na = CharacterVector(), int nskip = 0,
                                int n = 100) {
 
   XlsxWorkSheet ws(path, sheet);
@@ -43,7 +43,7 @@ CharacterVector xlsx_col_names(std::string path, int sheet = 0, int nskip = 0) {
 
 // [[Rcpp::export]]
 List read_xlsx_(std::string path, int sheet, RObject col_names,
-                RObject col_types, std::string na, int nskip = 0) {
+                RObject col_types, std::vector<std::string> na, int nskip = 0) {
 
   XlsxWorkSheet ws(path, sheet);
 
diff --git a/src/XlsxWorkSheet.h b/src/XlsxWorkSheet.h
index 8b0c59f2..a03f3f9e 100644
--- a/src/XlsxWorkSheet.h
+++ b/src/XlsxWorkSheet.h
@@ -63,7 +63,7 @@ class XlsxWorkSheet {
   }
 
 
-  std::vector<CellType> colTypes(const std::string& na, int nskip = 0, int n_max = 100, bool has_col_names = false) {
+  std::vector<CellType> colTypes(const StringSet& na, int nskip = 0, int n_max = 100, bool has_col_names = false) {
     rapidxml::xml_node<>* row = getRow(nskip + has_col_names);
     std::vector<CellType> types;
     types.resize(ncol_);
@@ -117,7 +117,7 @@ class XlsxWorkSheet {
 
   Rcpp::List readCols(Rcpp::CharacterVector names,
                       const std::vector<CellType>& types,
-                      const std::string& na, int nskip = 0) {
+                      const StringSet& na, int nskip = 0) {
     if ((int) names.size() != ncol_ || (int) types.size() != ncol_)
       Rcpp::stop("Need one name and type for each column");
 
diff --git a/tests/testthat/test-missing-values.R b/tests/testthat/test-missing-values.R
index 9e771131..7a4c388f 100644
--- a/tests/testthat/test-missing-values.R
+++ b/tests/testthat/test-missing-values.R
@@ -23,12 +23,24 @@ test_that("na arg maps strings to to NA [xls]", {
   expect_equal(df$y, c(NA, 1, 1)) # formula column
 })
 
+test_that("na arg allows multiple strings [xls]", {
+  df <- read_excel("missing-values.xls", na = c("NA", "1"))
+  expect_true(all(is.na(df$x)))
+  expect_true(all(is.na(df$y))) # formula column
+})
+
 test_that("na arg maps strings to to NA [xlsx]", {
   df <- read_excel("missing-values.xlsx", na = "NA")
   expect_equal(df$x, c(NA, 1, 1))
   expect_equal(df$y, c(NA, 1, 1)) # formula column
 })
 
+test_that("na arg allows multiple strings [xlsx]", {
+  df <- read_excel("missing-values.xlsx", na = c("NA", "1"))
+  expect_true(all(is.na(df$x)))
+  expect_true(all(is.na(df$y))) # formula column
+})
+
 test_that("text values in numeric column gives warning & NA", {
   expect_warning(
     df <- read_excel("missing-values.xls", col_types = rep("numeric", 2)),