BCIP Trust Associates III, a Bain fund that holds $5 million to $25 million of Mr. Romney’s retirement savings, is a partnership, not a blocker entity. But the Caymans-based fund appears to be using blockers to shield retirement savings from some taxation. Nonprofit investors in Bain funds, and in funds managed by many other investment firms, use blocker corporations to retain their nontaxable status with respect to unrelated business income, according to experts familiar with the practices of the firm and the industry.

“It’s to the tax advantage of Bain’s investors: avoid UBIT issues as you diversify your portfolio,” said Dean Zerbe, a longtime Senate Finance Committee investigator now in the private sector.

The fund may also have helped Mr. Romney individually, tax experts say. Like nonprofits, individual retirement accounts are subject to the unrelated business income tax, said Anne Moran and Suzanne McDowell, offshore-tax experts at Steptoe & Johnson, a law firm in Washington. If an I.R.A. invests in a partnership like Bain that has “debt-financed” investments, Bain’s specialty, income from those investments is subject to unrelated business taxes. For instance, an investor could put $1 in an I.R.A. and purchase a partnership interest of Bain Capital in the Cayman Islands, which, in turn, borrows $1,000 to buy 1,001 shares of a company near bankruptcy that Bain has just purchased. If the shares go to $100, the investor then has $99,100 after he pays off the $1,000 loan. Such a transaction would be walloped by the unrelated business tax if done on shore.

To reap the advantages of a partnership, the I.R.A. investment manager buys shares in a blocker corporation. The corporation then invests in the partnership, and investment gains are paid out as dividends, not subject to the tax, Ms. McDowell said.

Adding to the appearance that Mr. Romney has used such leveraged investments in his I.R.A. is its sheer size. Even if someone had contributed the maximum amount to an I.R.A. since 1975, when Congress created such tax-favored accounts, contributions would total roughly $100,000. And even if someone had contributed the maximum amount to an employer-provided retirement plan, then rolled that into an I.R.A., the total would be about $1.5 million.

Mr. Romney’s I.R.A. holdings, in 25 funds, total from $21 million to $102 million, according to his financial disclosure forms. To reach those totals, he may have hit the investment jackpot, or, more likely, his contributions were laden with debt and valued very low, tax experts said. As that debt was retired, the value of those contributions exploded.

The Romney campaign has not said whether the candidate’s I.R.A. investments are in a blocker entity, but they have come close. A campaign statement said Mr. Romney’s I.R.A. “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.”