Polyethersulfone (PES) ultrafiltration (UF) membranes with and without surface-modifying macromolecules (SMMs) were prepared and characterized in terms of the mean pore size and pore-size distribution, surface porosity, and pore density. The results demonstrated that both the mean pore size and the molecular weight cutoff (MWCO) of the SMM-modified membranes are lower than those of the corresponding unmodified ones. Membrane fouling tests with humic acid as the foulant indicated that the permeate flux reduction of the SMM-modified membranes was much less than that of the unmodified ones. Therefore, fouling was more severe for the unmodified membranes. Moreover, the dry weight of the humic acid deposited on the membrane surface was considerably higher for the unmodified membranes. (C) 2003 Wiley Periodicals, Inc.