European Commission President Jean-Claude Juncker spent 25,000 euros (£22,600) on a private plane to take a nine-person delegation to Rome, campaigners have discovered.
The use of the "air taxi" was among almost €500,000 (£451,000) in EU commissioners' transport and hotel bills during January and February 2016.
They were unearthed by Spanish-based Access Info and Belgian magazine Knack.
The Commission said the spending was within the EU's rules.
It was also scrutinized annually by the European Parliament, a spokeswoman added.
On Mr Juncker's trip from Brussels to Italy, he held meetings with then-Italian PM Matteo Renzi, the Senate president and the president of the chamber of deputies, among others.
Commission spokeswoman Mina Andreeva said the private jet was chartered because there was "no viable commercial plane available that would fit the president's agenda" and stressed the 2,927 euro (£2,650) per person cost of the flight.
By the BBC's Adam Fleming in Brussels
If the European Commission fears the return of the allegation that the EU is a giant gravy train (or a "gravy plane," maybe), then they did not show it.
Officials mounted a passionate defence of how much the 28 European commissioners spend on travel. It's their job to promote EU policies abroad! It's all within the rules! And the European Parliament approves the accounts!
Jean-Claude Juncker's €25,000 private plane to Rome was shared with eight officials who had no other way of getting there. And rules specify the maximum that can be claimed for a hotel room. For example, a night in Latvia cannot cost more than €105.
And is the commission in first class when it comes to the cost of diplomacy? Vice president Frans Timmermans spent the equivalent of £6,200 on travel in two months. In comparison, separate documents in the UK show that, say, the Brexit Secretary David Davis spent £10,576 over a similar period.
She also described flights on "air taxis" as "hard work", which includes "reading documents with your files and marking them", adding: "So I think you will be disappointed as to the travelling experience."
Among the other expenditure detailed, the foreign affairs chief Federica Mogherini spent €75,000 (£67,700) on a trip to Azerbaijan and Armenia.
Campaigners have spent several years trying to obtain detailed information about officials' expenses, with the commission arguing that collating it poses a big administrative burden.
UKIP MEP Nigel Farage, whose own expenses claims have come under scrutiny in the past, said Mr Juncker's use of a private plane over a commercial option was "outrageous".