In a study of undergraduate L2 students participating in a Content and Language Integrated Learning historical reasoning course, we examined students' changing performance on historical reasoning and how this was affected by their English reading and writing proficiency. Students engaged in written historical reasoning when answering a historical question by using sources and heuristics such as historical contextualization and corroboration. The course was designed based on principles likely to enhance historical reasoning and second language acquisition, and included an overt focus on language form. We found that students were able to reason historically at an emerging level of proficiency. The markers of historical reasoning present in their writing were neither significantly flawed nor highly proficient. A latent growth curve analysis was used to investigate the effect of students' English language proficiency on their source-based writing and the changes in their reasoning over the duration of the course. We found that students' English proficiency level did not predict either their reasoning or changes in their performance during the course. Students at differing levels of English proficiency improved similarly.