This article examines how master's students consult and process sources in source-based writing tasks in L1 and L2. Two hundred eighty master's students wrote a text in their L1 (Dutch) and L2 (English) at the beginning and end of the academic year. They wrote these texts based on three sources: a report, a web text, and a newspaper article. Their writing processes were registered using the Inputlog keylogging program. This allowed us to determine how much time the students spent reading the sources, when they did so, which sources they consulted most frequently, and how often they switched between the various (types of) sources. The quality of the texts was assessed holistically using pairwise comparisons (D-pac). Confirmative factor analysis showed three components to be relevant to describe source use in L1 and L2 writing: (a) initial reading time, (b) interaction with sources, and (c) the degree of variance in source use throughout the writing process. Individual text quality remained stable in L1 and L2 throughout the academic year. Structural equation modeling showed that the approach in source use, especially source interaction, is correlated with text quality, but in L1 only.