A group of about 130,000 (1981), the So are located on both sides of the Mekong River in Thailand and central Laos. Through extensive contact with the Sek and the Lao they have largely been assimilated into Laotian society. Included under the So are a number of subgroups including the So Trong, So Slouy, So Phong, So Tri, and So Makon.
Hickey, Gerald C. (1964). "So." In Ethnic Groups of Mainland Southeast Asia, edited by Frank M. LeBar, Gerald C. Hickey, and John K. Musgrave, 150. New Haven: HRAF Press.