Members of Congress routinely fail to report millions of dollars’ worth of costs that they are racking up on foreign trips, according to Treasury Department reports on the actual price tag of foreign travel.

Over the past 10 years, the cost of Congressional travel abroad has tripled, from about $6.4 million in 2001 to $19.4 million in 2008, dipping to $17.6 million last year, according to annual reports published by the Treasury Department covering the account that the State Department uses to pay travel costs for Members.

In total, according to the Treasury reports, the government has spent about $110.5 million on Congressional foreign travel since 2001.

That is $30 million to $40 million more than Congress detailed in routine reports published in the Congressional Record on foreign travel expenditures, though those reports are so rife with errors and inconsistencies that it is impossible to get a reliable total for any single year, let alone an aggregate for a decade.

A Roll Call tally of reports published in the Congressional Record indicates that the House and Senate combined have spent about $78 million on foreign travel since 2001. A study published by the Wall Street Journal last year came up with a total of about $73 million.

Whatever the year-by-year or cumulative total, two trends are clear: The amount disclosed by Congress in its public reports is far below the actual costs reported by the Treasury Department, and the total spent on foreign travel has skyrocketed in the past decade.

For example, in 2006 the Senate reported $3.84 million in foreign travel costs and the House reported $5.91 million, for a joint total of $9.75 million for the year.

According to the Treasury Department reports, the total spent out of the Congressional travel accounts that year was $13.94 million.

Neither the Treasury numbers nor the Congressional Record reports include the millions of dollars that the Pentagon spends each year on air travel for Members traveling abroad.

Roll Call reported earlier this year that Congressional foreign travel costs are paid out of a bottomless Treasury account that is replenished without any action by Congress. The account was established by a Korean War-era statute that allows the government to use excess foreign currency to pay the costs of Members and staff traveling abroad.

Travel arrangements are made by the State Department, which also approves the expenditures out of the account. But the State Department refused Roll Call’s request for details about the expenditures and said that a Freedom of Information Act request for details on the spending will take a year to process.

Congressional committees and the bipartisan leaders of both chambers are required by law to provide public reports on the expenditures made on their behalf from these accounts to cover the costs of foreign travel. These reports are full of errors and inconsistencies, some of which provide a hint to the millions of dollars that are missing from the disclosures.

Most of the Congressional reports include the per diems that Members and staff are entitled to during their travel, as well as the cost of their transportation (which is frequently zero because the Pentagon is providing the travel). But few of the reports include the on-the-ground costs that the State Department racks up in support of Congressional delegations and charges back to the foreign currency accounts.

The rare reports that detail these costs offer an intriguing peek into what else the foreign currency accounts buy Members on the road.

On Sept. 16, 2009, the House Homeland Security Committee published a report on a May trip of 22 people — including Chairman Bennie Thompson (D-Miss.) and Reps. Laura Richardson (D-Calif.), Henry Cuellar (D-Texas), Christopher Carney (D-Pa.), Jane Harman (D-Calif.) and Yvette Clarke (D-N.Y.) — to Brazil, Argentina, Peru and Panama.

The report includes a list of expenses for the delegation’s stay in Argentina, including $1,174 for a delegation “control room” in the hotel, $486 for “internet access @ hotel,” $2,000 for employee overtime and $270 for cell phones. The total of these extra costs for the Argentina stay is just under $5,500. These extras are routine on Congressional delegations abroad, but the report included no mention of similar costs at any of the other stops on the trip.

The expense detail for the Argentine visit is exceptionally rare in the Congressional reports; most of the reports exclude these costs entirely, and the ones that do include these expenses generally refer to them simply as “delegation costs” or “other purposes.”

Minority Leader John Boehner (R-Ohio) led a delegation to Panama for three days in December, and the report that he filed in the Congressional Record indicated only the costs of per diems for the 10 travelers. Asked about other costs of the trip, Boehner’s spokesman said, “We report what we’re supposed to report.”

Despite the wide gaps in reporting, the Congressional reports and the Treasury Department accounting both confirm the dramatic increase in the costs of foreign travel by Congress. According to the Treasury reports, the government spent about $2.3 million in both 2001 and 2002 to cover foreign travel costs for Senators and staff. That total jumped to about $6.6 million in both 2008 and 2009.

The House showed a similar jump in travel costs from 2001 to 2008, rising from $4.1 million to $12.7 million, but that number dipped to $11 million in 2009.

Several staff members in both chambers hypothesized that the increased costs may reflect Members’ growing interest in travel to Iraq and Afghanistan since 2001, but the reports don’t bear that out. Trips to war zones are conducted almost entirely in the embrace of the Pentagon, so those trips have almost no reported costs from the foreign currency accounts.

Earlier this year, Speaker Nancy Pelosi announced new rules on foreign travel intended in part to rein in costs. The California Democrat established tighter restrictions on when Members can fly business class and cracked down on Members and staff converting per diem payments to personal use.

In the Senate, travel policies are not centralized in leadership, and each committee sets its own rules. Frederick Jones, a spokesman for the Senate Foreign Relations Committee, says Chairman John Kerry (D-Mass.) “has insisted on an ongoing effort to reduce foreign travel costs as much as possible.” For example, “We have begun following State Department guidelines regarding coach travel.”

Jones said the committee achieved a 20 percent reduction in travel costs for the first six months of this year compared with the similar period in 2009, though Congressional Record reports indicate that the committee spent more in the first quarter of 2010 than in 2009. The committee’s second-quarter report will not be published until later this summer.

Alex Knott and Jessica Estepa contributed to this report.