One way for companies to not pay taxes in the U.S. is to move over sees. Despite a 2004 federal law that was made to put a stop to the practice, companies are still doing it.

Companies mention many reasons for moving, including expanding their operations and their geographic reach. However 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.

The companies that have moved recently include manufacturer Eaton Corp., oil firms Ensco International Inc. and Rowan Cos., as well as a spinoff of Sara Lee Corp. called D.E. Master Blenders 1753.

Eaton, a 101-year-old Cleveland-based maker of components and electrical equipment, announced in May that it would acquire Cooper Industries PLC, another electrical-equipment maker that had moved to Bermuda in 2002 and then to Ireland in 2009. It plans to maintain factories, offices and other operations in the U.S. while moving its place of incorporation-for now-to the office of an Irish law firm in downtown Dublin.

When Eaton announced the deal, it emphasized the synergies the two companies would generate. It also told analysts that the tax benefits would save the company about $160 million a year, beginning next year.

In June, the Internal Revenue Service tightened an exception that had allowed companies to move to countries in which they have substantial business activities. It will not prevent moves through a merger, such as Eaton's.

Lawmakers of both parties have said the U.S. corporate tax code needs a rewrite and they are aiming to try next year. One shared source of concern is the top corporate tax rate of 35%-the highest among developed economies. By comparison, Ireland's rate is 12.5%.

The Obama administration has proposed lowering the rate to 28%, while Republican rival Mitt Romney has proposed 25%.

Critics of the tax code also say it puts U.S. companies at a disadvantage because it taxes their profits earned abroad. Most developed countries tax only domestic earnings.

Tax reform needs to "put American businesses in the best position to compete in the global economy while adding U.S. jobs." said Sen. Max Baucus (D., Mont.), the Senate Finance Committee chairman, in a recent statement.

And House Ways and Means Chairman Dave Camp (R., Mich.) said in a recent statement that "comprehensive tax reform that lowers rates and transitions the U.S. to a territorial approach that is used by our global competitors is critical to making America a more attractive place to invest and hire."