A file with the XLSX file extension is a Microsoft Excel Open XML Format Spreadsheet file. It's a ZIP-compressed, XML-based spreadsheet file created by Microsoft Excel version 2007 and later.

When Microsoft rolled out Office 2007, the company made some fundamental changes to its file formats, updating them from binary files to XML-based packages. The default format for Excel 2007 ...

You can create XML data from a Microsoft Excel 2003 worksheet or workbook, but you'll need to do some manual steps to get the data to conform to valid XML format.