This research investigated the effect of modeling activities on grade 5 students' informal reasoning about a real-life issue. An instrumental case study was conducted with 17 students (7 female and 10 male) at a public middle school in Turkey. Data were collected through semistructured interviews and student worksheets. The students' informal reasoning patterns were analyzed using the informal reasoning pattern rubric; informal reasoning qualities were analyzed using the argumentation quality rubric. Furthermore, a content analysis was conducted to distill the relationship between informal reasoning and the students' models. Findings indicated that the students did not use emotive reasoning patterns in the post-interview, failed to create high-quality informal reasoning, and referred to the modeling activities in different components of their reasoning.