* Closed door hearing May 12 to 19
* Could help EU regulators determine size of fines
* EU Commission charged banks last year
* EU decision expected next year due to complex case
BRUSSELS, April 7 (Reuters) - Thirteen investment banks, including Citigroup, Goldman Sachs and Deutsche Bank, will fight EU charges of blocking exchanges' access to the credit derivatives market at a hearing next month, three people familiar with the matter said on Monday.
The closed-door hearing comes nine months after the European Commission accused the banks of preventing Deutsche Boerse and the Chicago Mercantile Exchange from entering the lucrative credit default swaps (CDS) business between 2006 and 2009.
The hearing before Commission competition and legal officials and their counterparts from national regulators could determine whether the banks face hefty fines which could be as much as 10 percent of their global turnover.
"The hearing will run from May 12 to 19," said one source, adding that it was a provisional date set by the EU competition authority which may change depending on circumstances.
Commission spokesman for competition policy, Antoine Colombani, declined to comment. European Competition Commissioner Joaquin Almunia has said he will let his successor rule on the case next year because of its complexity.
The other banks charged by the Commission include Bank of America Merrill Lynch, Barclays, Bear Stearns, BNP Paribas, Credit Suisse, HSBC, JPMorgan, Morgan Stanley, RBS and UBS .
The EU watchdog also cited financial data company Markit and the International Swaps and Derivatives Association (ISDA) in its so-called statement of objections or charge sheet sent to the group last year.
CDS, which are worth trillions of dollars, allow an investor to bet on whether a company or country will default on its bonds within a fixed period of time.
They were originally over-the-counter (OTC) or non-exchange traded contracts, but the market is shifting to exchanges since regulatory efforts to boost transparency began. The lack of transparency has been a key target of regulators following the 2007-2009 crisis.
(Reporting by Foo Yun Chee; Editing by Mark Potter)