Ieng Sary is the most prominent surviving Khmer Rouge leader - and is still viewed as an influential and respected figure in parts of Cambodia, reports the BBC's Guy Delauney from Phnom Penh.
He received a royal pardon 12 years ago after reaching a deal with the government that resulted in the eventual surrender of the Khmer Rouge.
His lawyers say this is why he should not be facing charges now. They will also argue that a trial would amount to double jeopardy.
The Vietnamese-backed forces which ousted the Khmer Rouge in 1979 tried Ieng Sary in absentia and found him guilty of genocide. That verdict was overturned by the pardon.
But Cambodians who survived Khmer Rouge prison camps feel particularly strongly about the former foreign minister, our correspondent adds.
Many of them were well-educated people who returned to the country after personal appeals from Ieng Sary to help rebuild Cambodia.
They were arrested on arrival, and thrown into brutal detention centres.
Ieng Sary's wife, former social welfare minister Ieng Thirith, has also been charged by the genocide court.
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