Many English language proficiency (ELP) tests used for university admissions and placement now include integrated writing tasks that require examinees to use external sources when writing. Integrated writing tasks improve test authenticity and impact, but they raise several validity questions, such as what academic language skills they engage and whether performance on these tasks varies with examinee ELP level. This study addresses these questions with reference to an integrated writing task from the Canadian Academic English Language (CAEL) Test that involves reading, listening, and writing in academic contexts. Responses by 59 students to one of the CAEL integrated writing tasks are analyzed in terms of various grammatical, discourse, sociolinguistic, strategic, content, and source use aspects and compared across ELP levels (high and low) and score levels. The findings indicate that both ELP level and score level had significant effects on most writing features examined in the study, except for syntactic complexity. Additionally, except for syntactic complexity, all writing dimensions examined in the study were significantly associated with writing scores. The findings and their implications for the validity argument of source-based writing tasks are discussed.