* 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
By Foo Yun Chee
BRUSSELS, April 7 (Reuters) - Thirteen investment banks,including Citigroup, Goldman Sachs and DeutscheBank, 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 EuropeanCommission accused the banks of preventing Deutsche Boerse and the Chicago Mercantile Exchange from entering thelucrative credit default swaps (CDS) business between 2006 and2009.
The hearing before Commission competition and legalofficials and their counterparts from national regulators coulddetermine whether the banks face hefty fines which could be asmuch 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 competitionauthority which may change depending on circumstances.
Commission spokesman for competition policy, AntoineColombani, declined to comment. European CompetitionCommissioner Joaquin Almunia has said he will let his successorrule on the case next year because of its complexity.
The other banks charged by the Commission include Bank ofAmerica 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 andthe International Swaps and Derivatives Association (ISDA) inits so-called statement of objections or charge sheet sent tothe group last year.
CDS, which are worth trillions of dollars, allow an investorto bet on whether a company or country will default on its bondswithin a fixed period of time.
They were originally over-the-counter (OTC) or non-exchangetraded contracts, but the market is shifting to exchanges sinceregulatory efforts to boost transparency began. The lack oftransparency has been a key target of regulators following the2007-2009 crisis. (Reporting by Foo Yun Chee; Editing by Mark Potter)