A detailed study on concentration polarisation during microfiltration (MF) was carried out in order to improve control on cake layer formation. Concentration polarisation is reduced by several back-transport mechanisms, however, for particles in the size range 0.5-30 mum shear-induced diffusion is the predominant one. Shear-induced diffusion is caused by the hydrodynamic interactions between particles in shear flow, which are strongly dependent on the particle size. Because of this, the polydispersity of suspensions is expected to have a large effect on cake layer formation. Although this aspect will be of considerable practical relevance, it has only received minor attention in literature. In this work, the effect of particle size distribution on MF was investigated by studying the steady-state flux of bidisperse suspensions with a ratio of particle radii of 3. The flux appeared to be completely determined by the smaller particles; it remained almost constant between a volume fraction of 0.2 and I of small particles. This trend in the steady-state flux was in good agreement with calculations based on a shear-induced particle migration model for polydisperse suspensions. Since polydispersity is a common property of suspensions, the present work may contribute to optimisation of flux and selectivity in MF processes, which depend largely on adequate control of cake layer formation. This can be achieved by implementation of the developed model in process control routines.