If you filed as married filing jointly then both of you are responsible for the backed taxes and the IRS can take a refund you would receive. You could look into equitable innocent spouse relief though.
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The IRS will take your refund for 2012 and apply it to the taxes you and your ex-husband still owe for 2009. The only possibility for avoiding this situation is if you can qualify for innocent spouse or injured spouse relief. You haven't stated any facts that would be relevant to that determination; however, in general it is very hard to get innocent/injured spouse relief unless you were well and truly unaware of, and had no reason to suspect, that there would be unpaid taxes in 2009.
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