The interaction between Suwannee River humic and fulvic acids and ultrafiltration membranes is characterized by direct adsorption measurements. The coated membranes are also characterized by the following parameters : hydrophobicity determined by contact angle measurements, pore zeta potential computed from streaming potential measurements, and surface chemical composition obtained from X-ray photoelectron spectroscopy (XPS). The results show that the adsorption capacity is smaller for fulvic acid than for humic acid. Fulvic acid competes with calcium phosphate compounds for adsorption sites on the membranes. Low pH and, in some cases, high calcium concentration increase the adsorption of humic substances on the membranes. The adsorption capacity decreases with decreasing (more negative) pore zeta potential and with increasing hydrophilicity. Upon adsorption of humic substances, the membranes become more hydrophilic, and the apparent pore charge becomes less negative. Our results support a model of humic substances adsorption in which pore adsorption sites are preferentially filled prior to other sites on the membrane surface.