The aim of this study is to understand and to characterise the influence of the montmorillonite fraction on the behaviour of several mixtures of kaolin and montmorillinite, ranging from pure kaolinite to pure montmorillonite. We show that beyond 35% in weight, montmorillonite is responsible for the global behaviour of the mixture. At the global level, the influence of montmorillonite becomes apparent through the fact that one-dimensional compressibility path, in the (log sigma(v)-e) plane, loses its linearity. At the local level, MIP results show that the compressibility, for any of the studied mixtures, results in variations in inter-aggregate porosity. The qualitative analysis of SEM images explains the macroscopic behaviour, showing that the more flexible particles of montmorillonite form a structured network surrounding the kaolinite particles. When the percentage of montmorillonite exceeds 35%, the role of this phase becomes major, imposing in particular the particles orientation mechanisms.