“This is a classic example of how good tax planning avoids taxes until you want to pay taxes on the money,” said Martin Lobel, a Washington lawyer and chairman at Tax Analysts, a provider of information for tax specialists.

Even many Americans with sophisticated tax advisers “can’t take advantage of that kind of loophole,” Lobel said.

According to an August 2011 financial disclosure, Romney’s individual retirement account included a stake valued at $5 million to $25 million in something called BCIP Trust Associates III.

Regulatory filings show that the partnership, related to Romney’s career at the corporate buyout firm Bain Capital, is registered in the Cayman Islands.

The offshore arrangement could have spared Romney a form of U.S. tax that can apply even to individual retirement accounts, experts say.

IRAs are widely used investment vehicles that allow people to save money and postpone paying taxes on it until their golden years. But even within the shelter of tax-exempt entities such as IRAs or nonprofit organizations, some investments can trigger taxes by throwing off profits known as “unrelated business taxable income.”

That includes returns on debt-financed investments. Hedge funds and private equity funds are especially likely to trigger the tax because they often seek to magnify their returns by using borrowed money.

To spare investors the tax bite, hedge funds and private equity firms often set up corporate shells known as “blockers” in havens such as the Cayman Islands, experts said.

David Miller, a lawyer at the Cadwalader firm who works with private-equity firms and hedge funds, said Bain might have formed entities in the Cayman Islands to accommodate clients such as foundations and endowments — tax-exempt organizations trying to steer clear of the tax on unrelated business income.

In the past week, since news reports about the Cayman connection emerged, the Romney campaign has been on the defensive about the issue. In a statement Tuesday, the campaign said the former Massachusetts governor’s IRA “uses investment structures just as those commonly used by charities and pension funds, including union pension funds, to maintain their tax exempt or tax deferred status.”

“The Romneys’ investments in funds established in the Cayman Islands are taxed in the very same way they would be if the Romneys held their shares of the fund investments directly in the U.S. rather than through a Cayman fund,” the statement said. It was not clear whether that part of the statement applied to the IRA.

It is unclear how, if at all, Romney’s taxes were affected by the offshore arrangement.

A Bain spokesman did not respond to questions Tuesday.

Bart Mallon, a lawyer at Cole-Frieman & Mallon who sets up offshore companies, said such arrangements appeal to foreign investors. Many foreigners prefer investing through an offshore company because they don’t want to file U.S. tax returns or expose themselves to potential Internal Revenue Service audits, he said.

“Unfortunately, when people hear the term ‘offshore,’ they automatically think that there’s, you know, that people . . . are trying to get away with something,” Mallon said.

“But it’s not really getting away with something. It’s just working within the tax laws as they’re currently written.”