The purpose of this study was to investigate how concept cartoons, together with other diagnostic and scaffolding tools, could be used in formative assessment, to stimulate talk and argumentation among students in small groups, as part of peer-assessment and self-assessment; and to provide diagnostic feedback about students' misconceptions to the teacher for teaching towards conceptual change. Two classes of Primary 5 and 6 students worked in small groups to discuss the opposing viewpoints posed by the cartoon characters, using scaffolding tools to guide their discussions and to evaluate, challenge, and document each others' ideas. Students also used drawings to depict their ideas. The conversation from one group was audio-taped. These tools provided a record of students' thinking in a form that was accessible to the teacher for monitoring and feedback purposes. Findings showed dialogic talk and interactive argumentation among students where they made their reasoning visible. Students' assertions and questions had formative potential as they encouraged exploratory and reflective discourse by drawing upon each others' ideas. The teacher's discursive practices, as well as her role in designing scaffolding structures for supporting 'assessment conversations' when using concept cartoons and in devising strategies that take into account students' conceptual and epistemic thinking, are emphasised. (Contains 3 figures.)