Recent studies have shown that membrane surface morphology and structure influence permeability, rejection, and colloidal fouling behavior of reverse osmosis (RO) and nanofiltration (NF) membranes. This investigation attempts to identify the most influential membrane properties governing colloidal fouling rate of RO/NF membranes. Four aromatic polyamide thin-film composite membranes were characterized for physical surface morphology, surface chemical properties, surface zeta potential, and specific surface chemical structure, Membrane fouling data obtained in a laboratory-scale crossflow filtration unit were correlated to the measured membrane surface properties, Results show that colloidal fouling of RO and NF membranes is nearly perfectly correlated with membrane surface roughness, regardless of physical and chemical operating conditions. it is further demonstrated that atomic force microscope (AFM) images of fouled membranes yield valuable: insights into the mechanisms governing colloidal fouling, At the initial stages of fouling, AFM images clearly show that more particles are deposited on rough membranes than on smooth membranes. Particles preferentially accumulate in the "valleys" of rough membranes, resulting in "valley clogging" which causes more severe Aux decline than in smooth membranes.