Background/purpose – Argumentation plays an essential role in higher-order activities and in the communication of mathematical knowledge. Although the purposes of argumentation have piqued the interest of many researchers, few have simultaneously investigated the quality of argument in students’ mathematical problem solving.

Materials/methods – In the present case study, 41 middle school students in Indonesia solved the argumentative task. The students’ responses were analyzed for the quality of their arguments. As it was the goal of this research to assess the quality of their argument in mathematical problem solving, the responses were coded with the Polya method and the CER model.

Results – The results revealed that more than half of the students misunderstood the problems provided, which were not followed by the appropriate evidence and reasons.

Conclusion – These results indicated that further handling is needed to improve the quality of arguments in students and to develop the importance of activities that support students in explaining, justifying, and correcting their reasons, during the mathematical argumentation process.