Reflection has recently been emphasized as a constructive pedagogical activity. However, little attention has been given to the quality of reflections that students write. In this study, we explored the reflections that students make about their knowledge organization as part of a formative learning activity. More specifically, we assessed the knowledge structures of Grade 11 physics students and their instructors using pathfinder networks (PFnets). Each student's knowledge structure was compared with the instructors' averaged knowledge structure in order to identify student misconceptions. As an intervention, students were asked to write reflections on the discrepancies between their knowledge structure and their instructors' averaged knowledge structure. The students' reflections were divided into the following three categories depending on the type of knowledge constructed in those reflections: (1) conceptual, (2) procedural, or (3) declarative. Evidence was provided by the study that reflection was an effective means of improving students' knowledge structure. However, conceptual reflections were the most effective, followed by procedural and declarative reflection. Implications for formative classroom assessment are discussed.