Although there is a wealth of research on the use of the first language (L1) in English as a foreign language (EFL) classrooms, there is as yet very little research of this kind in classrooms where the prime pedagogical objective is to teach academic content through English as a second language (English medium instruction; EMI). It is important to begin filling this gap because a purported aim of content-based programs is to expose students to large quantities of the target language. We investigated the practices of five EMI teachers in a Chinese university and measured the reactions of their students both quantitatively and qualitatively. Our findings show that these teachers switched to the L1 rarely (although with considerable differences among the teachers) and mostly to explain both simple and complex concepts in their academic disciplines. Although students were unperturbed by the switches to the L1, some felt that the teacher could have made more of an effort to explain it in L2 first.