Thaksin himself will attend for a Buddhist ceremony, as Thais and Cambodians celebrate the New Year.
The group rates Cambodia the 20th-most corrupt nation in the world, putting it in the company of Guinea, Kyrgyzstan and Yemen.
Defense attorneys have said that the woes over the two cases highlight corruption in the tribunal’s judicial process.
Several Asean members, notably the Philippines and Vietnam, have overlapping claims to the sea with China and Taiwan.
China claims ownership of much of the South China Sea, along with four ASEAN states, including the Philippines.
Host President Hun Sen focused on economic issues in his opening remarks Tuesday.
Burmese President Thein Sein told reporters in Phnom Penh Tuesday the elections had been a success.
Cambodia had left the contentious issue off the agenda, following a visit by Chinese President Hu Jiantao over the weekend.
The displaced at Phnom Bath say they are vulnerable to heavy winds and rainstorms, and they fear poisonous snakes and insects.
Burma’s political reform process has been a high-profile objective for the 10 member bloc of the Association of Southeast Asian Nations.
Hu’s visit, the first from a Chinese head of state in 12 years, comes as Phnom Penh is hosting five days of Asean meetings.