Although feedback is a popular topic in educational research, the question of how and on what conditions feedback in mathematics affects learning seldom has been addressed. In this study, we investigated: (1) whether process-oriented feedback in mathematics leads to greater interest and higher achievement development compared to social-comparative feedback; (2) whether students' perception of feedback with regard to usefulness and competence support mediates these effects; and (3) whether the impact of feedback is moderated by students' mastery approach goal orientation. To answer these research questions, 146 ninth-grade intermediate school students in Germany were randomly assigned to both feedback conditions. Results of path analyses revealed (1) no significant total feedback effects on interest and achievement development, but (2) indirect effects on the development of interest via perceived competence support and usefulness, and on achievement development via perceived usefulness, as well as (3) a moderation effect of mastery approach goal orientation on the impact of feedback on perceived usefulness. (Contains 3 figures and 2 tables.)