The aim of the presented study is to examine the learning benefit of rubrics as part of the process of formative assessment. Teachers often have difficulties explaining to students the criteria for solving complex mathematical tasks like problem solving or reasoning. However,the new Swiss standards of mathematics require all teachers to place more emphasis on mathematical processes. Standard-oriented rubrics help teachers to provide feedback for learning based on clear criteria. Our pilot study consists of a small sample of four primary classes. We used quantitative methods to measure the students’ competency in mathematical reasoning. In addition, we interviewed the teachers and groups of students to evaluate our intervention. Using Rasch modelling, we were able to validate the four levels of our rubric, but the reliability of the students’ competency level measures was rather low due to the few complex items. The qualitative content-analysis of the interviews shows that all students have generated a basic understanding of the competency. They gained the insight that reasoning is not just wrong or right, but it can be done at different competency levels. The students have learned to self-assess themselves and the teachers felt supported by the rubric when giving feedback.