Summary: This chapter presents an example of mathematics within a multidisciplinarity context, which took place recently in Danish upper secondary school. The case’s topic was modelling a system of chemical equilibrium in a solution of molecules and ions. The teacher deliberately focused on the intertwined mathematical modelling and chemical modelling, as a means to realize a multidisciplinary teaching perspective of the two subjects. The students’ written reports were analysed with the aim to study different aspects of their perceptions of modelling, as a result of multidisciplinary teaching. Means and obstacles for support of modelling competency by multidisciplinary teaching are discussed.