More big U.S. companies are reincorporating abroad despite a 2004 federal law that sought to curb the practice. One big reason: Taxes.

Companies cite various reasons for moving, including expanding their operations and their geographic reach. But tax bills remain a primary concern. A few cite worries that U.S. taxes will rise in the future, especially if Washington revamps the tax code next year to shrink the federal budget deficit.

"We want to be closer to where our clients are," says David Prosperi, a spokesman for risk manager Aon plc, which relocated to the U.K. in April.

Aon has told analysts it expects to reduce its tax rate, which averaged 28% over the past five years, by five percentage points over time, which could boost profits by about $100 million annually.

Since 2009, at least 10 U.S. public companies have moved their incorporation address abroad or announced plans to do so, including six in the last year or so, according to a Wall Street Journal analysis of company filings and statements. That's up from just a handful from 2004 through 2008.