A group of 29 European journalists is taking the European Parliament to court in the hope of making MEPs' expenses more transparent.
The complaint has been filed with the European Court of Justice (ECJ), whose rulings are binding EU-wide.
MEPs' allowances "should not be allowed to remain hidden from the European public", said the group's leader, Slovenian journalist Anuska Delic.
They say MEPs' pay and expenses total more than €474m (£332m) annually.
They are particularly concerned about MEPs' general allowances, which they calculate to be almost €40m (£28m) annually.
A statement from the group said "the complete absence of monitoring of MEPs' general allowances... raises serious doubts that other spending by the 751 MEPs is monitored in a transparent, responsible and verifiable manner".
Several expenses scandals have hit the European Parliament in recent years.
The 29 journalists say they decided to go to the ECJ in Luxembourg after the European Parliament denied them access to the information on MEPs' expenses.
The anti-corruption watchdog Transparency International (TI) says it supports the group's initiative and has submitted its own access to documents request for information on MEP allowances.
"The European Parliament lags far behind other parliamentary bodies, such as Sweden and the UK, when it comes to the transparency of parliamentary expenses," said TI's EU director Carl Dolan.
One very expensive part of all MEPs' work is their monthly return trip from Brussels to Strasbourg, for plenary votes. MEPs themselves have called on the EU to make Brussels the "single seat" for their work.
On her website Green MEP Jean Lambert lists an MEP's monthly expenses - standard for all 751 - as:
- Staff allowance - €21,379 (covering all costs of personal assistants)
- General Expenditure Allowance - €4,320 (covering things like constituency office rent, phone bills)
- Daily allowance - €306 (for attendance at parliamentary sessions in Brussels or Strasbourg)
- Travel allowance - MEPs' tickets are reimbursed, plus a €4,264 annual allowance for trips outside usual places of work.
MEPs' monthly salaries are set at just over €8,000, before tax.