This article describes the interaction between mathematics and language, based on an analysis of how individual learners solve word problems in English as a foreign language (L2). It reports on a study conducted to investigate how the L2 influences mathematical thinking and learning in the process of solving word problems and how the construction of meaning unfolds. The research generated the Integrated Language and Mathematics Model (ILMM), which facilitates the description of the interplay between mathematics and language. The empirical results show, inter alia, that CLIL learners tend to use the given text more profoundly for stepwise deduction of a mathematical model, and conversely, mathematical activity can lead to more intense language activity. Furthermore, effective mathematical activity depends on successful text reception, and problem solving in a L2 provides additional opportunities for reflection, both linguistically and conceptually. The ILMM makes a major contribution to conceptualising content and language integration.