Research investigating the cognitive effect of bilingualism on mathematical learning began just over 25 years ago and has progressed from there, although research focusing on the Asia-Pacific region is still quite limited. This paper presents details about a current project in this area. CARE's Riel Maths research project investigates ethnic minority student achievement in mathematics by testing the hypothesis that ethnic minority students in the early school years who attend bilingual schools where the mother tongue and national language, Khmer, are the languages of instruction, are at an advantage in terms of mathematical growth compared with ethnic minority students attending Khmer language only schools. The Riel Maths Project is a longitudinal panel study with the research being carried out over 4 years, from 2009 to 2012.