Integrated writing tasks, which require students to incorporate information from multiple sources into their writing, are gaining attention in language assessment and writing instruction due to their alignment with the expectations of academic and professional writing. Understanding what aspects of integrated tasks are perceived as challenging by L2 students is necessary for informing teaching and assessment of integrated writing. This exploratory study investigated how L2 students approach integrated writing tasks utilising both listening and reading sources, with a focus on the challenges they experience when writing their source-based essays. Employing inductive analysis, this research examined stimulated verbal reports obtained from twelve English language learners. The findings revealed that students' challenges were primarily related to source use and task fulfilment demands. Students also articulated other challenges related to the organisation and structure of integrated essays as well as the grammatical, lexical, and mechanical citation issues that had emerged while composing essays. The findings have important implications for test developers and L2 writing instructors in developing diagnostic rubrics targeting specific writing features and designing instructional activities.