Using matrix assisted laser desorption ionisation mass spectrum (MALDI-MS), this study reports the observations of the fouling distribution and composition along the membrane channel after the membranes were subjected to ultrafiltration of protein mixture solution in a crossflow module with and without spacer. In the fouling layer on a fully retentive membrane, the protein components with high molecular weight has higher presentation after 2 h of filtration and the presentation reduced to be lower than the smaller components after 6 It of filtration due to protein exchange and displacement phenomena in deposition layer caused by the differences in structure and diffusivity of different components. The protein exchange and replacement in the deposition layer was also observed on partial retentive membrane using a sequential fouling procedure. Fouling distribution along the membrane channel with spacer inserted in the module was more uniform and the flux was higher than that without spacer despite higher protein deposition observed in some cases. (c) 2006 Elsevier B.V. All rights reserved.