As an important and a challenging source-based writing task, synthesizing offers rich opportunities to explore the connections between reading and writing. In this article, we report findings from a qualitative study of two Chinese students' learning experiences with academic synthesis writing in a university ESL composition course. Specifically, we discuss how the two students' understanding of synthesis and sources influenced their synthesis writing practices and how they perceived the connections between their reading strategies and synthesis writing processes. Our results reveal that the students' understanding of synthesis and the functions of sources played a crucial role in learning to synthesize, as did their ability to use rhetorical reading strategies to complete this new literacy task. We argue that whether second language (L2) students understand the complex reading-writing relationships underlying synthesizing is crucial for their successful textual production. These findings carry valuable implications for understanding reading and writing connections and teaching L2 source-based writing.