Official policy in post-apartheid education is aimed at redressing linguistic inequity in schooling by promoting the 11 official languages of South Africa through mother tongue instruction. However, since the life chances of children are inextricably linked to the language of power, many parents believe that their children would benefit from instruction in English. Consequently, there has been an ever-increasing demand for English to be the language of instruction in schools. Utilising case studies of Grade 4 pupils at three desegregated schools, this research sought to determine whether Grade 4 learners, whose mother tongue is not English, would be able to express their conceptual mathematical understanding better when tested in English or in their mother tongue. The major finding was that although many non-first-language English speaking Grade 4 learners are challenged by having to acquire abstract subject knowledge through a non-mother tongue medium, they face an even greater challenge when tested in their mother tongue. (Contains 1 figure and 2 tables.)