The Microsoft Excel spreadsheet application stores spreadsheets in a OLE2 compound file based format. The name of the files typically ends with .xls or .xlb.
OpenOffice.org's Documentation of the Microsoft Excel File Format
Includes a description of the binary file format of Microsoft Excel, including all available Excel versions (for Windows) from Excel 2.1 to the current Excel XP. [PDF]
Last update:January 16, 2014 at 5:37:43 UTC