BOSTON – Sometimes the joy of gay marriage is followed by the pain of gay divorce, but Peter Zupcofska is there to help.

In Boston, he has become known as "the master of gay divorce." And for a $25,000 retainer, he can lead couples through the intricacies of how to best break up. Zupcofska said divorces present gay couples with a new opportunity "to make things right" when a relationship ends.

"Divorce is a critical part of marriage," he said.

Gay divorces are bringing many new wrinkles to the field of family law, raising questions that are sure to be litigated for years to come. While no one tracks how many gay couples have divorced in Massachusetts, attorneys in the Boston area are competing for cases in the budding cottage industry.

The cases are complicated because of vast differences in state and federal law. In short, federal law trumps state law and can eliminate benefits that a state might want to extend.

Tax issues are a good example.

Gay marriages are not recognized by the Internal Revenue Service, meaning spouses cannot take advantage of federal tax breaks, including the basic deduction of a dependent.

Gay spouses are eligible for alimony and child support, but such payments cannot be deducted from federal taxes. Gay spouses must file federal tax returns as individuals, and only one of them can claim a child as a dependent.

And if a couple splits and one spouse wants to transfer a house to the other, it can result in a higher federal tax that wouldn't apply to divorcing heterosexual couples.

"If Peter were married to Paula, there would be no tax consequences," Zupcofska said. "With Peter married to Paul, you have a capital gains tax. And the capital gains tax is 15 percent." (That would result in a $75,000 tax on a $500,000 gain.)

Pension issues also get messy.

Because Massachusetts recognizes gay marriages, state workers can get their spouses covered by pensions. But gay federal employees aren't eligible for Social Security under their partner's plan, as heterosexual couples are. And any other retirement plan governed by federal law does not have to recognize a gay spouse.

Zupcofska, who is gay and married, doesn't like to leave things to chance: He has a prenuptial agreement with his spouse, and he advises his clients to do the same.

Zupcofska said the poorer spouse can suffer when a marriage breaks up if the issues aren't ironed out in advance. Without a prenuptial agreement, he said, the primary breadwinner can walk away with more money in his pocket, even if it has been a long-term relationship of 20 years or more.

Courts will also have to address issues involving the rights of parents and who's responsible for raising children.

For example, if two gays are married and both are recognized as the parents of a child in Massachusetts, that won't necessarily apply in a state that doesn't recognize gay marriage. Zupcofska said that raises questions regarding who would be legally liable for child support or what happens if children inherit money from their grandparents: Can someone be a grandchild in one state and not in another?

Getting divorced can also be tricky.

Two lesbians who married in Massachusetts moved to Rhode Island and tried to divorce there, but the Rhode Island Supreme Court wouldn't allow it because the state defines marriage as a union only between one man and one woman. As a result, one of the women had to move back to Massachusetts to establish residency and qualify for a divorce.

Lisa Cukier, another Boston attorney who handles divorce cases, said that legal issues stemming from gay divorces are now being handled on a case-by-case basis. But she predicted that at some point, a gay rights group will challenge the federal law against gay marriage and the Supreme Court will be forced to resolve the differences.

But Joyce Kauffman, a Cambridge attorney who represents divorcing gays and lesbians, said that most gay activists believe now is not the time to try to get a gay marriage case before the nation's highest court.

Many activists hope their chances will improve if Illinois Democratic Sen. Barack Obama wins the presidency and appoints more liberal justices who might view gay marriage more favorably.

In the meantime, Cukier said gays and lesbians who marry must be vigilant to protect their rights.

Call Rob Hotakainen, McClatchy Washington Bureau, (202) 383-0009.