The purpose of this study was to: characterize the interaction of humic substances with common ultrafiltration membranes; correlate physicochemical properties of humic substances and membranes to adsorption of humic substances on membranes; and examine the changes in membrane surface characteristics caused by adsorption of humic substances. Results show that a number of surface science tools could be used to characterize membranes, humic substances, and adsorption of HS on membranes. Streaming potential measurements showed clear differences in zeta potential of the membranes. As the mass of adsorbed HS increased, the zeta potential of the membrane pores increased (became less negative). Contact angle measurements on clean membranes also showed sensitivity to membrane composition, although the results of this measurement are expected to be clouded by the effect of surface roughness and porosity. Nevertheless, as the mass of adsorbed HS increased, the contact angle measurements for specific membranes showed a clear increase in membrane hydrophilicity. Even though HS are very water soluble, our measurements of octanol-water partition coefficient showed that HS become more hydrophobic as pH decreases. Humic acid adsorbs on the membranes studied to greater extent than fulvic acid. Calcium is very effective in enhancing adsorption of humic acid on two types of membrane.