An Atomic Force Microscope (AFM) has been used to quantify directly the adhesive force between a colloid probe and two polymeric ultrafiltration membranes of similar MWCO (4000 Dal but different materials (ES 404 and XP 117, PCI Membrane Systems (UK)), The colloid probe was made from a polystyrene sphere (diameter 11 mu m) glued to a V shaped AFM cantilever. Measurements were made in 10(-2) M NaCl solution at pH 8. It was found that the adhesive force at the ES 404 membrane was more than five times greater than that at the XP 117 membrane, As it allows direct quantification of particle/membrane interactions, this technique should be invaluable in the development of new membrane materials and in the elucidation of process behaviour.