This study examined whether instruction in genre knowledge enriches students' feedback on each other's writing, resulting in better writing quality. In total 140 sixth-grade students (age 11-13) participated in the study. Two approaches to peer response with additional instruction were compared. In one condition, students were taught specific genre knowledge (SGK). In another condition, students were taught general aspects of communicative writing (GACW). Both groups were compared with a baseline control group. Students were randomly assigned to the conditions. Results showed strong effects of the SGK condition outperforming the other conditions on text quality of four posttest writing tasks. Video recordings of students commenting on each other's first drafts showed that the students in the SGK condition gave significantly more attention to the functions taught than students in the GACW condition. This finding supports the interpretation that knowledge about the genre-specific functions was actually used to improve texts.