Unless you have a final decree of legal separation, then you and your estranged husband were still married for 2012 and neither of you can file as single. Instead, not only may you file as married filing separately, you must file as married filing separately.
However, be prepared for the IRS to question your return; they will compare your return to his return - not the least because you're required to put his name and social security number on your return even if you're filing as married filing separately - and when they see you filing as married filing separately and him filing as single they'll want to know why.
My answer does not constitute legal advice and may not be relied upon by anyone for any purpose and does not constitute an attorney/client relationship or an offer to form such a relationship. This disclaimer is intended to be fully compliant with the requirements of Treasury Department Circular 230 and the terms thereof are fully incorporated by reference. If you wish to consult with me please contact me at dana@nytaxcounsel or visit my website at www.nytaxcounsel.com
Married filing separately...
Total Mobility Law is an international law firm that lets companies do global business with the knowledge and confidence they need to comply in any country. Our answers on this site do not constitute legal advice, nor do they establish an attorney-client relationship. The only thing that can do that is a signed Engagement Letter and Fee Agreement, which you can get by contacting us through www.totalmobilitylaw.com.
Sign up to receive a 10-part series of useful information and legal advice about the divorce process.