Abstract: Interactive dialogue within feedback episodes is essential for developing primary school students' mathematical reasoning competence. Our goal was to better understand the nature of the associations between observed dialogue, teachers' formative feedback, and students' mathematical reasoning. We applied a two-step approach, first constructing a video-analysis instrument for assessing the quality of interactive dialogues and then combining the interaction data with student and teacher questionnaire data from 804 students in 44 fifth and sixth grade primary school classes. The quality of the observed dialogues predicted class differences in students’ self-efficacy for explaining but not in their reasoning competence, which was predicted by perceived formative feedback.