What tool are you using to create the ribbon?

I use Office Ribbon Editor and if I want a ribbon to work in Word 2007, I need to ensure the CustomUI Part is RibbonX12 as well as having the namespace as per Greg's posts. If your CustomUI Part is RibbonX14 then it probably won't validate correctly with the older namespace.

Note that if you set up the ribbon for Word 2007 it will also work for Word 2010 and 2013 - forward compatibility is alive and well. It appears I could include different ribbons for different versions of Word but that is way more effort than I would entertain.

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Andrew Lockton
Chrysalis Design, Melbourne Australia