Designed to gauge the actions, skills, and strategies that students use to identify, extract, and synthesize information from multiple source materials, integrated writing tasks are particularly challenging for ESL writers. Previous research tended to focus on a limited number of integrated tasks, mostly summaries of one or two sources, thus limiting our understanding of L2 learners' performance on more demanding integrated writing tasks. This study, part of a larger validation study of the the Canadian Academic English Language (CAEL) Test, extends this line of research by examining the cognitive processes that ESL learners engage in when responding to a source-based argumentative writing task and how, if at all, these cognitive processes vary depending on students' English language proficiency (ELP). Eight undergraduate ESL students at two levels of ELP (high and low) provided stimulated recalls (SRs) about their writing processes immediately after completing an integrated writing task that involved listening to a lecture, reading a passage about a related topic, and writing an argumentative essay using ideas from both sources. Analyses of the SRs and in-depth analysis of the writing process of one high and one low ELP students revealed the various cognitive and metacognitive processes and strategies that the participants used and the language and discourse aspects they attended to. Generally, participants with higher ELP tended to interact with the sources and task and to engage in planning and organizing, generating and retrieving, and revising and editing more frequently than did participants with lower ELP. The findings and their implications for the teaching and assessment of source-based writing in Canadian L2 writing classrooms and beyond are discussed.