This allowed him to do fieldwork on languages of Laos, including Hmong and Yao (Hmong-Mien family), Khmu/Khamou and Lamet (Austroasiatic/Mon-Khmer), as well as Phu Noi/Phou-Noy (Sino-Tibetan).

He mainly did fieldwork in Thailand and Burma (Myanmar) in the 1980s, studying Wa, Lawa, Palaung, Mon and Nyah Kur; in Vietnam and Laos in the 1990s, studying Viet-Muong (also known as Vietic) languages, and the Tai languages and writing systems of northern and central areas of Vietnam, including the Lai Pao writing system of Vietnam, which was close to falling into oblivion.

[1] He has published extensively about his findings on numerous languages of Laos, Thailand, Burma/Myanmar, and Vietnam, in journals such as Mon-Khmer Studies, Cahiers de Linguistique Asie Orientale, and Diachronica.

Michel Ferlus's main discoveries relate to the effects of monosyllabicization on the phonological structure of Southeast Asian languages.

[3] Phenomena such as the spirantization of medial obstruents, which resulted in a major historical change in the sound inventory of Vietnamese,[4] are also part of the broad set of changes—originating in monosyllabicization—that swept through East/Southeast Asia.