Mastering the skill of writing from multiple sources is essential for university students. Against today's multilingual backdrop, students not only need to engage with various sources but may also encounter materials in different languages. However, most existing research on source-based writing has focused on monolingual contexts, leaving us with a limited understanding of how students integrate cross-linguistic sources in their writing. To address this gap, we conducted a preliminary exploration of language use in the context of writing an English text based on a series of Chinese (L1) and English (L2) sources among 33 university students in Hong Kong. By utilising multiple methods (i.e., writing logs, questionnaires, and interviews), we examined students' language use as reflected in their access to sources in different languages and their self-reported writing processes. The results did not show a significant difference in the use of L1 and L2 in integrating sources, despite the final written text being in L2. Additionally, we found that the two languages dynamically interacted with each other, supporting different stages of the writing process. The study also provides pedagogical implications for source-based writing in a bilingual context. Copyright © 2024 Published by Elsevier Ltd.