Romney didn't take travel deductions related to his income from public speaking, said Ken Brier, a tax attorney in Needham, Massachusetts, who described his returns as "pretty squeaky clean."

"He's gone pretty light on his deductions," Brier said. "I guess he doesn't want anyone to question it."

The returns also demonstrate how, using sophisticated estate planning, Romney has been able to give millions of dollars to his children free of estate and gift taxes, because of a legal structure known as a "grantor trust."

Romney established three trusts to which he contributed assets. The campaign said his children were listed as beneficiaries, though didn't specify of which trusts. The income generated by the trusts triggers a tax obligation for Romney. By picking up that tax bill, he found a legal way to transfer money to his children free of gift taxes, said Mark G. Bosswick, managing partner at Berdon LLP, an accounting firm in New York.

'Great Technique'

In addition, any appreciation of the stock after it was donated to the trusts grows free of gift and estate tax, assuming a gift tax was calculated at the time of the donation. The estate tax won't be applied to the appreciation of assets that are no longer in his estate.

"It's a great technique because not only are you doing the initial gift, but by him paying the taxes on the trusts' income each year he's making additional gifts to the trust beneficiaries free of gift tax," Bosswick said. "Plus, any appreciation of the investments will not be subjected to estate and gift tax."

The Romneys paid $232,989 because of the alternative minimum tax, which is designed to prevent people from avoiding taxes legally. That parallel tax system doesn't eliminate the preference for investment income.

The tax returns shed no new light on Romney's considerable individual retirement fund, worth as much as $101.6 million, according to a financial disclosure he filed in August. It's unclear why the value is so high, given that contribution limits to IRAs are relatively low.

Cayman Investments

Kleinbard speculated that Romney claimed low values on carried interest in various Bain funds that he contributed to the IRA.