Nitti, who works with high-income clients, said the Romneys' trusts have investments in such places as the Cayman Islands. Even if the Romneys didn't make those investment decisions, he said, "there's some sophisticated tax planning going on."

The 2010 return shows that the Romneys' blind trusts have invested in an array of assets based around the world, including the Caymans. Brad Malt, a partner at Ropes & Gray LLP in Boston who operates the family's blind trusts, said those funds are fully taxable and fully reported.

"When we make an investment in a Cayman fund, we're presented with investment documents," he said. "The sponsor has already chosen where the fund is to be organized."

Such offshore funds attract overseas investors who don't want to file with the IRS, and also incur no or minimal corporate tax in the overseas jurisdiction.

Tax Advantage

"If the fund was in England, they'd have to pay tax there and in the U.S.; this way they only pay it here," said Bradley Smallberg, a CPA at Smallberg Sorkin and Co. LLP in Melville, New York.

The Romneys had a bank account in Switzerland, the return shows. The account for one of the trusts at UBS AG held about $3 million and it has since been closed, Malt said.

Malt, who began working as the Romneys' trustee in 2003, said during that time he was unaware of any IRS audit of the couple's tax returns.

Romney's income probably didn't place him among the top 400 taxpayers. For the past several years, reaching that list required an adjusted gross income of more than $100 million.

In 2008, John McCain's presidential campaign released the senator's tax returns for the previous two years, 2007 and 2006, said Douglas Holtz-Eakin, who served as policy director for the Arizona Republican's campaign.

Those tax documents, released on April 18, 2008, showed that McCain earned $405,409 in 2007 and paid $118,660 in federal taxes. He gave $105,467 to charity, the records show.