By Telis Demos 

The keepers of the Dow Jones Industrial Average have made it clear that companies that do business in the U.S. but are incorporated overseas aren't eligible for the stock-market benchmark. The new definition will preclude companies that have moved abroad through tax-lowering inversion deals from being in the blue-chip index.

A committee overseeing the Dow index this month adopted new language in its methodology requiring member companies to be incorporated and headquartered in the U.S. In addition, the U.S. must be a company's biggest revenue-generating country.

While those criteria were historically understood, they hadn't been formally written down as the benchmark's rules, said David Carlson, director of U.S. and Canadian equity indexes at S&P Dow Jones Indices, which oversees the Dow index.

"Considering this is one of the oldest indices, and always considered to be a purely U.S. index, we wanted to maintain that legacy," said Mr. Carlson, who added the move had been in the works for some time. S&P Dow Jones Indices is a part of McGraw Hill Financial Inc.

The Dow tracks 30 stocks from different sectors, ranging from drug maker Merck & Co. to media company Walt Disney Co. The stocks are selected and criteria are set by a committee that doesn't use a quantitative method but instead looks at the reputation, growth and prominence of the companies. Wall Street Journal editors are among the committee's members, as is Mr. Carlson.

The new language comes at a sensitive time, as a number of large U.S. companies are in the process of incorporating outside the U.S. through mergers that allow them to cut their corporate tax rates. In such deals, known as inversions, a U.S. firm buys an overseas company and relocates its legal home to a lower-tax country such as the U.K. or Ireland.

Since 2009, at least 35 U.S. companies have relocated overseas for tax purposes, according to a study by S&P Capital IQ. Only 25 did so in the preceding eight years, the study found.

The Dow Jones Industrial Average methodology also states that companies must be in the S&P 500 index, a broader benchmark of the U.S. market, to be considered for inclusion. Although the S&P 500 also is limited to U.S. companies, it doesn't disqualify companies that move their legal headquarters abroad if they continue to meet other criteria.

At present, 24 companies in the S&P 500 are headquartered outside the U.S., and four more are incorporated overseas but still have main offices in the U.S., according to S&P Capital IQ.

Seven companies in the S&P 500 have recently announced inversion deals, according to S&P Capital IQ. None are in the Dow.

One Dow constituent that recently considered an inversion was Pfizer Inc. The drug maker's bid this year to buy U.K.-based AstraZeneca PLC was rebuffed, and in recent weeks it explored a tax-lowering takeover of Ireland-based Actavis PLC, the Journal reported Tuesday.

The Dow's move to explicitly exclude non-U.S.-based companies could in theory have share-price implications for companies that choose to do inversions, as some index funds may be forced to sell those stocks.

The Dow, however, has $33 billion in assets tied to the index, compared with the S&P 500, which has about $1.9 billion.

The Dow's move comes as other index providers have decided to make it easier to retain companies that reincorporate overseas.

Most of the major index providers "have methodologies that make sure [companies] won't suddenly be excluded from American indices and moved to [foreign] ones," said Spencer Bogart, an analyst at ETF.com, which tracks indexes and exchange-traded funds. "Inversions won't have any real impact" on stock ownership.

Mat Lystra, senior research analyst for Russell Investment Group's stock indexes, said the company--whose indexes include the Russell 3000 index of small-capitalization companies-- in 2010 adopted more flexible criteria for determining a company's home country.

Before the change, he said, "incorporation was the only thing we looked at. But it's not sufficient anymore. Where to incorporate has become a strategic decision for a company.

"We're trying to parse out companies where it is just a strategic decision, like an inversion, but also want to acknowledge when a business and its operations have changed," he said.

Write to Telis Demos at telis.demos@wsj.com

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