This study drew upon activity theory to understand university students' learning of English in tertiary English-medium instruction (EMI) contexts in China. It investigated seven Chinese university students' resource-mediated learning of English in EMI programs with an analysis of in-depth interview responses and stimulated recalls. The findings illustrated varieties and dynamics in terms of the students' English language learning practice. The interaction among the students' perceived content-oriented learning goals, the artifacts (e.g., textbooks, PowerPoint slides, journal articles) and community resources (e.g., classmates) they possessed, the rules for learning in their EMI programs (e.g., time limits, social norms), and the roles they played in the learning activity (e.g., content learner, English user, passive English learner, Chinese learner) explained why they learned English in this way. The study informs the formulation of educational language policy, as well as the development of curricula and pedagogical schemes for EMI programs. (C) 2020 Elsevier Ltd. All rights reserved.