Nuon Chea and Khieu Samphan will attend a preliminary hearing at the UN-backed war crimes tribunal to request release from pre-trial detention. They and two other senior figures face charges of genocide for their parts in the deaths of around two million Cambodians between 1975 and 1979.
The elderly defendants have all been in detention since 2007.
Nuon Chea was the second in command to the Khmer Rouge leader, Pol Pot, and is accused of devising the policies which caused the deaths of hundreds of thousands of people. The 84 year old, who was known as "Brother Number Two", is arguing that the Tribunal had no right to extend his pre-trial detention.
Similar points will be made by the former head of state, Khieu Samphan, and ex-social affairs minister, Ieng Thirith, although she is not thought to be attending the hearing in person.
More at the BBC.