The cake formation during filtration of a bidisperse polystyrene solution containing 3.3 and 5.7 mu m particles has been studied. Microfiltration membranes are used in treating feed suspensions with a wide variety of particles, of different types and sizes. These different particles determine the properties of the cake formed during operation. Using an optical system, the local properties of the cake have been determined while simultaneously following the fouling with global methods like pressure and flow measurements. The cake thickness was measured and coupled with the resistance data. A minimum cake porosity was observed at 50% number fraction of larger particles. The measured specific cake resistance is compared to a predicted specific cake resistance obtained using the Kozeny-Carman relation incorporating a correction for the bidisperse particle distribution. The Kozeny-Carman relation showed a similar trend to the experimental results, with lower absolute values. This was suggested to be due to better ordering or compaction of the cake in some regions, which resulted in lower experimental porosities in comparison to the theoretical porosities. (C) 2010 Elsevier B.V. All rights reserved.