Fouling of a negatively charged polyurethane surface by substances typical of those occurring in the wet end of paper machines was studied by QCM-D and AFM. The aim was to evaluate the influence of surface forces, adsorption kinetics, viscoelasticity, and morphology of the adsorbed layers on the fouling process, which is discussed in terms of three steps:(1)contacting of deposits with the surface,(2)adherence of the deposits to the surface, and(3)ability of the adsorbed layer to collect further deposits. Substances studied were pitch emulsions, latex, AKD, kaolin and talc dispersions, cationic starch, and other soluble polymers. pH was 5.5-7. Adsorption tendencies depended, as predicted by the DLVO theory, on the ionic strength and valence of simple ions. Kaolin or polystyrene latex adsorbed only above critical salt concentrations. Strongly coagulated pitch emulsions(large aggregate size)adsorbed to a lesser degree than weakly coagulated emulsions(smaller aggregate size), apparently due to the slower diffusion rate of the larger aggregates.