diff --git a/file.go b/file.go
index 034eddab..f22e80c9 100644
--- a/file.go
+++ b/file.go
@@ -7,6 +7,7 @@ import (
 	"io"
 	"os"
 	"strconv"
+	"strings"
 )
 
 // File is a high level structure providing a slice of Sheet structs
@@ -203,6 +204,14 @@ func (f *File) MarshallParts() (map[string]string, error) {
 		return parts, err
 	}
 
+	// Make it work with Mac Numbers.
+	// Dirty hack to fix issues #63 and #91; encoding/xml currently
+	// "doesn't allow for additional namespaces to be defined in the root element of the document,"
+	// as described by @tealeg in the comments for #63.
+	oldXmlns := `<workbook xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main">`
+	newXmlns := `<workbook xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main" xmlns:r="http://schemas.openxmlformats.org/officeDocument/2006/relationships">`
+	parts["xl/workbook.xml"] = strings.Replace(parts["xl/workbook.xml"], oldXmlns, newXmlns, 1)
+
 	parts["_rels/.rels"] = TEMPLATE__RELS_DOT_RELS
 	parts["docProps/app.xml"] = TEMPLATE_DOCPROPS_APP
 	// TODO - do this properly, modification and revision information
diff --git a/file_test.go b/file_test.go
index 8b276c98..d71cf34d 100644
--- a/file_test.go
+++ b/file_test.go
@@ -636,8 +636,11 @@ func (l *FileSuite) TestMarshalFile(c *C) {
 	c.Assert(parts["xl/_rels/workbook.xml.rels"], Equals, expectedXLSXWorkbookRels)
 
 	// workbook.xml
+    // Note that the following XML snippet is just pasted in here to correspond to the hack
+    // added in file.go to support Apple Numbers so the test passes.
+    // `xmlns:r="http://schemas.openxmlformats.org/officeDocument/2006/relationships"`
 	expectedWorkbook := `<?xml version="1.0" encoding="UTF-8"?>
-<workbook xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main"><fileVersion appName="Go XLSX"></fileVersion><workbookPr showObjects="all" date1904="false"></workbookPr><workbookProtection></workbookProtection><bookViews><workbookView showHorizontalScroll="true" showVerticalScroll="true" showSheetTabs="true" tabRatio="204" windowHeight="8192" windowWidth="16384" xWindow="0" yWindow="0"></workbookView></bookViews><sheets><sheet name="MySheet" sheetId="1" xmlns:relationships="http://schemas.openxmlformats.org/officeDocument/2006/relationships" relationships:id="rId1" state="visible"></sheet><sheet name="AnotherSheet" sheetId="2" xmlns:relationships="http://schemas.openxmlformats.org/officeDocument/2006/relationships" relationships:id="rId2" state="visible"></sheet></sheets><definedNames></definedNames><calcPr iterateCount="100" refMode="A1" iterateDelta="0.001"></calcPr></workbook>`
+<workbook xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main" xmlns:r="http://schemas.openxmlformats.org/officeDocument/2006/relationships"><fileVersion appName="Go XLSX"></fileVersion><workbookPr showObjects="all" date1904="false"></workbookPr><workbookProtection></workbookProtection><bookViews><workbookView showHorizontalScroll="true" showVerticalScroll="true" showSheetTabs="true" tabRatio="204" windowHeight="8192" windowWidth="16384" xWindow="0" yWindow="0"></workbookView></bookViews><sheets><sheet name="MySheet" sheetId="1" xmlns:relationships="http://schemas.openxmlformats.org/officeDocument/2006/relationships" relationships:id="rId1" state="visible"></sheet><sheet name="AnotherSheet" sheetId="2" xmlns:relationships="http://schemas.openxmlformats.org/officeDocument/2006/relationships" relationships:id="rId2" state="visible"></sheet></sheets><definedNames></definedNames><calcPr iterateCount="100" refMode="A1" iterateDelta="0.001"></calcPr></workbook>`
 	c.Assert(parts["xl/workbook.xml"], Equals, expectedWorkbook)
 
 	// [Content_Types].xml