An increasing number of writing programs and assessments are employing writing-from-sources tasks in which reading and writing are integrated. The integration of reading and writing in such contexts raises a number of questions with regard to writers' use of sources in their writing, the functions these sources serve, and how proficiency affects discourse synthesis. To answer these questions, the current study used a mixed-method approach with a group of undergraduate students in a Middle Eastern university. One hundred forty-five students worked on a reading-to-write task and completed a questionnaire. In addition, nine students participated in think-aloud writing sessions and follow-up interviews. The qualitative data yielded initial patterns, which were explored further by quantitative analysis of relevant questionnaire items using descriptive statistics and chi-square tests. Results showed that source use serves several functions including generating ideas about the topic and serving as a language repository. Score level affected text comprehension, especially at lower levels, but was not found to relate to the source use functions. Based on these results, a number of recommendations for writing instructors and language testers are provided. (Contains 9 tables.)