This is what I found:
http://en.allexperts.com/q/Desserts-747/un-used-cake-mix.htm
HTH
It's been said by many that after you mix a cake mix, it should be baked immediately and not saved for later due to the ingredients. I'm sure that the thread JanH has given will definitely help you because she is the best at finding everything!! Thanks JanH 
I've actually had good luck freezing unused cake batter. I put it in a plastic container, seal with plastic wrap to make sure there is no air, then put the container's lid on for extra protection. Label it with type and date, and stick in the coldest part of the freezer.
I add that leftover bit when I'm making a cake (of the same flavor) that requires just a "little bit more" batter than full boxes of mix. I've never had an issue and the cakes turn out just fine.
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