Myanmar's junta-appointed foreign minister, Than Swe, is due to join the talks, two sources with knowledge of the meeting told Reuters.
But some ASEAN members have declined to attend and others are only sending junior officials.
ASEAN FRUSTRATIONThe military took over in Myanmar in 1962, isolating it for decades until a tentative opening up began in 2011.
Malaysia's foreign minister has also declined to attend the Thai talks, saying it remained supportive of the efforts being undertaken by Indonesia.
Cambodia on Monday said Foreign Minister Prak Sokhonn, who last year served as an ASEAN special envoy to Myanmar, would be represented by his deputy.
Persons:
Aung, Athit, Don Pramudwinai, Don, Retno Marsudi, Suu Kyi, Prak Sokhonn, Panu, Ananda, Kanupriya Kapoor, Kay Johnson, Robert Birsel
Organizations:
Protesters, Embassy, REUTERS, Monday, Association of Southeast Nations, ASEAN, Myanmar's, Reuters, Thai PBS, Indonesian, Peace, Myanmar, ASEAN Chair, Malaysian, Cambodian, Thomson
Locations:
Myanmar, Bangkok, Thailand, BANGKOK, ASEAN, Indonesia, Suu, Cambodia, Ananda Teresia, Jakarta, Rozanna, Kuala Lumpur