In order to design effective instruction and feedback for synthesis writing on both writing processes and products, a clear insight into synthesis writing processes underlying a high-quality synthesis text is crucial. That is why this study, as one of the first, examines the use of sources during synthesis writing processes, and its effect on text quality. The writing processes of 294 Dutch secondary students (grade 10-12) were logged using keystroke logging software Inputlog. Two different synthesis text genres were investigated of which three source-related process measures were analysed: the relative time spent in the sources, the transitions per minute between the sources, and the transitions per minute between the synthesis text and the sources. First, the study explored the effect of temporal distribution and genre (argumentative or informative synthesis) on the writing process, providing insights into the distribution of source-related writing activities over the process intervals and the possible influence of genre on this distribution. Secondly, the individual source-related process measures were linked to text quality. Thirdly, via polynomial regression analyses, the various source-related activities and their temporal distribution were taken into account in an integrated way to identify patterns of effective source use. These patterns vary across genre and explain a considerable amount of variance in the data (24.6% for argumentative synthesis texts, 16.2% for informative synthesis texts). Our findings can be used to develop process-oriented feedback, giving students an insight into their synthesis writing process.