(MONEY Magazine) – This filing season, some 48 million married couples signed joint tax returns. In so doing, each spouse swore that "to the best of my knowledge and belief, [the return was] true, correct and complete." It's an oath that some spouses will live to regret.

Despite recent liberalizations in the tax law, signing a joint tax return still makes each spouse liable for back taxes, interest and penalties the Internal Revenue Service may later assess in an audit. You are on the hook for the entire amount, even if your spouse earned all of the income and claimed all of the write-offs--and even if you've subsequently divorced and stipulated in your decree that your ex-spouse must pay any tax bills arising from prior joint returns.

True, provisions in the 1998 IRS reform law make it somewhat easier to get out from under your spouse's--or your ex-spouse's--tax debts. For starters, the law relaxed some of the requirements to qualify as a so-called innocent spouse, a designation that essentially absolves you of your spouse's tax sins. They also created two new levels of indulgence, known inelegantly as equitable relief and separate liability. As welcome as the new protections are, however, it's important to note that they can be very limited and are inherently complex. Below are the three options you face when your spouse's tax mess lands in your lap.

Innocent spouse relief

Under the new rules, you can apply for innocent spouse relief for any item that is challenged on a joint return. In contrast, the old law let you seek such relief only for "grossly erroneous" entries that had generally resulted in a tax understatement of more than $500.

But the new rules do not repeal the highest, and often insurmountable hurdle: If you claim to be an innocent spouse, the burden of proof is on you to show that you did not know and had no reason to know of your spouse's tax hijinks. Say, for example, the IRS asserts that a married couple failed to report income on a joint return and attempts to collect back taxes, interest and penalties from the now ex-wife. The woman may be able to prove that she did not know her ex-husband was hiding income. But her claim for innocent spouse relief is likely to be rejected if her lifestyle was out of line with the income shown on the returns. That's because a reasonably educated person is expected to know that an elaborate lifestyle is impossible to maintain on a modest income.

Equitable relief

This category was established for a spouse who is not clueless enough to be "innocent" but can nevertheless make the case that being faulted for the other spouse's misdeeds would be unfair. Moreover, equitable relief is intended for filers who reported their taxes correctly but did not pay the balance due. Assume, for instance, that you filed a joint return showing you owed $15,000. Also assume that you and your spouse paid half of the bill and took out a loan to cover the remaining $7,500. If, without your knowledge, your spouse blew the loan proceeds at the blackjack table, the IRS could agree not to come after you for the debt.

Separate liability

If you are divorced or legally separated or have been living apart from your spouse for at least the past 12 months, you can cut your tax ties to your (soon-to-be) ex by reworking your prior joint returns to specify which income and write-offs belonged to you alone.

The task can be very tricky and thus would require the help of a tax pro if your taxes are even somewhat complicated. You must file an election with the IRS stating that you are requesting separate liability, and your assertions are subject to challenge by your ex-spouse and the IRS.

But if all parties agree that you've correctly divvied up the returns, the IRS can't hit you up for the tax on an item attributable to your ex unless it proves you had "actual knowledge" of the issue--a very tough standard. For example, if the IRS wanted you to pay back taxes on unreported income from one of your ex's accounts, it would need evidence such as bank records showing that you withdrew money from the hidden stash. "The Tax Court will probably take your word for it unless there is a smoking gun," says Chris Rizek, former associate tax legislative counsel for the Treasury Department.

Clearly, even if you manage to extricate yourself from the responsibility for a joint return, you're likely to pay a high price, emotionally and financially. Your best course of action is not to rely on the new rules to help you out, but to go into joint filing as you should go into marriage itself--with your eyes wide open.

If you have any doubts about your spouse's tax moves, ask for an explanation. If you're not satisfied with the answer, file your own return, using the status known as "married, filing separately." Bear in mind that separate filing usually results in a higher overall tax bill than joint filing. But look on the bright side: If you iron out your differences with your spouse, you have three years from the time you file to amend a separate return to a joint return.