Collaborative learning is a well-established approach to elicit reasoning. The ability to solve mathematical modelling problems depends much on the ability to reason, because mathematical modelling problems are usually presented to students through texts. Decisions have to be made at all stages of the mathematical modelling cycle, for instance on assumptions, simplifications and feasibility of the model. In this paper we present an observation instrument to study collaborative modelling. There are three core components: collaborative learning, mathematical modelling and the language that students use while working together.