The aim of this study was to contribute to the understanding of the effectiveness of formative assessment interventions by analysing how the quality of programme delivery affects students’ mathematics achievement and interest. Teachers (n = 17) implemented formative assessment in their ninth-grade mathematics classes and provided their students (n = 426) with written process-oriented feedback. Four feedback characteristics (number of feedback comments, specificity, feedback at self level, social reference norm) and two types of embedment of feedback in the instructional context (focus on feedback utilisation, focus on performance evaluation) were evaluated. Multilevel regression analyses revealed no significant effects of feedback characteristics on interest but negative effects of number of feedback comments and specificity on achievement in mathematics. Positive effects on mathematics achievement and interest were found when feedback was embedded in instruction and had emphasis on feedback utilisation. Students’ interest also was affected positively when performance evaluation was stressed.