WASHINGTON – The law is full of exceptions, and when it comes to conflicts of interest involving personal investments, Congress found a big one: It didn't write a law.

While Congress prescribed a strict conflict-of-interest law for the judiciary, which prompted Supreme Court Justice Samuel Alito to remove himself from a punitive-damages case involving Exxon Mobil, it didn't apply the same test to lawmakers.

That has allowed Sen. Kay Bailey Hutchison to oppose bills that would hurt Irving-based Exxon, in which Ms. Hutchison owns stock valued between $100,000 and $250,000, according to disclosure reports. The reports don't require lawmakers to record the exact value of an asset. Mr. Alito's stock in Exxon is valued within the same range.

Ms. Hutchison, R-Texas, also owns between $50,000 and $100,000 of stock in Chevron, which, like Exxon, opposes proposed legislation to impose a "windfall profits" tax on oil companies.

In December, Ms. Hutchison put a "hold" on a bill – a tactic that allows one senator to stop legislation – that would have repealed tax breaks for Exxon, Chevron and three other major oil companies. The measure would have raised $9.4 billion for the U.S. Treasury over 10 years.

Ms. Hutchison, who says tax hikes would drive up gasoline prices for consumers, is likely to lead a push to oppose Democrats' efforts to revive the measure later this month.

Under Senate rules, Ms. Hutchison can vote on matters affecting Exxon because personal investments generally don't constitute a conflict of interest. Yet for officials in the judicial and executive branches, they can.

Under federal law, judges must disqualify themselves if they have "a financial interest" in a matter before them or "in a party to the proceeding."

Mr. Alito sat out the Exxon Valdez oil-spill case without saying why. Most justices don't state their reasons for such decisions, but the law that guides them is quite clear.

"Congress decided we ought to have a nice, bright-line rule," said Ronald D. Rotunda, a law professor at George Mason University and expert in legal ethics. "The rule in the federal statute says that if you own even one share in a company, you must disqualify yourself."

Some experts say different rules apply to lawmakers because legislative policy is general in nature, whereas judges and federal agencies often decide matters that affect only a few parties.

If lawmakers were forced to recuse themselves over stocks, they might make very few votes, said Fred Wertheimer, president of Democracy 21, a nonpartisan group devoted to campaign finance and political ethics.

Senate rules generally trust that senators act in the public interest, even if they have a financial stake in legislation, said Wilson R. Abney, a former staff director of the Senate Ethics Committee. A senator's personal finances are generally considered irrelevant unless there is proof that a legislator's action was motivated by personal enrichment.

"The Senate rule is not necessarily as broad as the concept of a conflict of interest," Mr. Abney said. "A conflict of interest exists any time an official has a financial interest in the decision he's making."

The rules are tighter for executive-branch officials, who can't write regulations if they own stock worth more than $25,000 in an affected company.

"Members have, in effect, created tougher rules for the executive branch than they have for themselves," Mr. Wertheimer said. "The circumstances are different, so there is a justification for it. But there probably is not a justification for the complete laxness of those [Senate] rules."

Ms. Hutchison isn't the only lawmaker who has owned valuable shares of Big Oil. Sen. Tom Coburn, R-Okla., who joined her in blocking the tax provisions in last year's energy bill, reported owning between $1,001 and $15,000 worth of Exxon stock in his 2007 financial disclosure form. The companies kept the tax breaks because the measure didn't pass the Senate.

Texas Sen. John Cornyn also opposed the tax policy, but the Republican doesn't own any individual energy stocks, said Brian Walsh, Mr. Cornyn's spokesman.

Sen. Jeff Bingaman, Democratic chairman of the committee with jurisdiction over energy policy, and his wife reported owning Exxon stock worth between $51,000 and $115,000 in last year's report. The couple has since sold their stock in Exxon, said Jude McCartin, Mr. Bingaman's spokeswoman.

Mr. Bingaman, D-N.M., voted last year to repeal the tax breaks.

In the House, Rep. Pete Sessions, R-Dallas, owns Exxon stock but is less of a player on energy issues than Ms. Hutchison. Mr. Sessions owns between $15,001 and $50,000 of Exxon stock; his two dependents own shares worth between $100,001 and $250,000, said Emily Davis, Mr. Sessions' spokesman.

"He's strongly advocated for domestic production, regardless of the company," Ms. Davis said.

Lawmakers report their stocks and other sources of outside income in financial disclosure statements. The reports are meant to advertise potential conflicts of interest, although senators make those judgments themselves.

Recusals are rare, Mr. Wertheimer said.

Members of Congress submitted new reports to the Senate Office of Public Records on Thursday, but they aren't made public until June. Matt Mackowiak, a spokesman for Ms. Hutchison, said her investments in Exxon and Chevron didn't change from last year.

Most voters don't look at the reports as they evaluate a legislator's work. For that reason, some experts argue that lawmakers should announce whether they own stock in a company affected by a bill.

"The member would ... say, 'notwithstanding this financial investment, I believe it's important to the public interest that I take this action,' " Mr. Abney said.

Shares of Exxon closed at $91.30 Thursday.