In school mathematics, mathematical reasoning with an emphasis on language is considered an important competence. A student’s competence to reason in mathematics requires specific reading and writing skills, but suitable activities to support these skills are difficult to find. In this thesis, an Educational Design Research study has been conducted to support Grade 4 students’ reasoning competence when reading and writing texts in school mathematics. During a 15-week period, the reading comprehension strategies of prediction, clarification, questioning and summarization found in Palinscar and Brown’s Reciprocal Teaching model were implemented. To analyse what unfolded in the classroom and what comprised the everyday teaching and learning activities as well as what needed to change, a conceptual framework of coordinating theories and concepts from the Reciprocal Teaching model and Systematic Functional Linguistics was applied. The results from the intervention show that all four reading comprehension strategies in the Reciprocal Teaching model were essential, and the combination of them facilitated a change in the students’ mathematical reasoning. However, the strategies needed to be modified for school mathematics, and the teacher’s method of scaffolding became essential for further developing the students’ reasoning competence.