The regional and cultural closeness of Botswana and South Africa, as well as differences in their political histories and language policy stances, offers a unique opportunity to evaluate the role of language in reading outcomes. This study aims to empirically test the effect of exposure to mother tongue and English instruction on the reading performances of grade 4 students in Botswana and South Africa using the 2011 pre-Progress in International Reading and Literacy Study (prePIRLS) and propensity-reweighted regression adjustment. The empirical findings provide evidence of significant positive effects of earlier and continued exposure to mother tongue instruction. Further attention and research needs to be directed at understanding the implications of language in education policy for achieving additive rather than subtractive bilingualism.