They are, by their own account, a perfectly happy couple. Yet every winter for three years, David and Angela Boyter of Ellicott City, Md., flew off to the Caribbean to get a quickie divorce.

In so doing, they enjoyed romantic vacations while saving a large chunk of money on income taxes.

The Boyters have divorced three times and remarried twice in the last five years just so that when tax filing time rolled around, they could reap the benefits of lower tax rates for single persons. It is a process that the Internal Revenue Service calls a "tax sham."

For years, the IRS has bristled at the fact that some couples were doing this, just as many couples such as the Boysters were upset that they had to go to such extreme lengths to get the tax break they felt they deserved. Next month, however, the IRS and the Boyters are going to U.S. Tax Court in the first case testing whether such "tax divorces" are legal or invalid for tax purposes.

Since 1969, the tax laws have forced married working couples to pay higher taxes than two single people living together earning the same amount, according to Angela Boyter.

So she and her husband have taken advantage of a section of the tax code that says marital status for tax purposes is determined on Dec. 31 of each year. They made sure they were divorced at that time.

"We save a lot of money. We don't pay a penalty tax," she said of their plan. "But I do want to be married. And I intend to get remarried as soon as Congress passes something to change the tax laws."

The tax divorce notion was born in the Boyter household at a dinner party in 1975, when the couple was consoling a recently divorced friend, Angela Boyter recalled.

"At least I'll be paying less taxes," she remembered their friend saying. "You guys should get divorced, too."

Boyter said she and her husband just laughed, but as the year went on, they stopped laughing and started checking the rates for quickie divorces in Haiti.

In December 1975, they flew off to a vacation of sun-drenched days, delicious French dinners and gorgeous scenery in Port au Prince for their first annual $350 divorce.

"I had no personal qualms about the divorce," Angela Boyter said yesterday. "Except I guess I was afraid of the IRS more than anything else."

In January 1976, the couple filed tax returns and then remarried in the Howard County courthouse. It was not until they had gone through the process once more, that a local newspaper reporter discovered the story. Suddenly they were the center of public attention.

Letters started coming in from around the country from couples interested in the "how-tos" of the tax divorce, Boyster said.

There also were a couple of "crank letters" to the Boyters' employer -- the federal government -- asking that the couple be fired, she said. Angela Boyter is procurement officer for a federal agency and her husband is physicist for another federal agency, though neither will divulge precisely where they work.

The SRS also apparently learned of what the Boyters were doing through the publicity and audited their tax returns for 1975 and 1976. The IRS then cited them for more than $3,100 in unpaid taxes for the two years, according to the Boyters' attorney Marvin Garbis.

The Boyters had paid about $17,600 in taxes during those two years, filing as single individuals, according to Angela Boyter.

IRS district counsel Charles Zarubin said that for tax purposes the courts will not recognize the divorces because "nothing really changed. They maintained their marital relationship and remarried."

IRS spokesman Dominic Laponzina said the divorces were nothing more than "a sham" to get around the law. But Garbis, the couple's attorney, said yesterday that the only test for filing as a single or a married individual is "are you married on Dec. 31."

"The IRS is trying to close a tax opportunity for modern-thinking people," said Garbis. "This is a legitimate way to avoid a penalty that the Internal Revenue Code inadvertently put on being married."

Boyter said that in a major revision of the tax code in 1969, the tax rates were set up so that married couples combining incomes were forced to pay higher taxes than two single people combining their incomes.

The structure hits hardest at a husband and wife earning nearly equal amounts, as Boyter and her husband do. Both earn about $30,000 annually.

Also, the standard deduction for a single person last year was $2,200, so two individuals living together could take a total deduction of $4,400 on their tax returns. A married couple, however, filing jointly could take only a $3,200 deduction.

IRS spokesman Laponzina does not deny that these facts are correct.But he said that the problem only became apparent in recent years as the number of married working couples grew and more wives began to earn higher salaries.

"In the past, the pressure was always from single people wanting the tax laws changed because they claimed they were being penalized," said Laponzina.

Boyter said she and her husband have been lobbying with letters to Congress since 1969 to get the tax laws changed and have used the tax divorce only as a temporary solution.