Streaming potential measurements have typically been performed on reverse osmosis (RO) and nanofiltration (NF) membranes using simple solution chemistries that are representative of those found in natural waters and wastewaters. In this investigation, streaming potential measurements were performed on RO membranes using actual membrane feed waters. Tertiary wastewater effluents which serves as membrane feedwater at the Orange County Water District (Fountain Valley, California) and river water from the Brazos River (Brazos, Texas) were used. Additionally, the effects of glutaraldehyde (a membrane storage agent) and atrazine (a pesticide) on membrane surface charge were evaluated. The pesticide was evaluated in the presence and absence of humic acid. Results showed a more positive zeta potential in the presence of the Brazos River water, most likely due to the presence of divalent cations (Ca2+ and Mg2+). For the tertiary wastewater effluent, adsorption of organic acids in the wastewater effluent may have been the cause of the more negative zeta potential in,the low pH range and the presence of divalent cations may have been the cause of the more positive zeta potential in the high pH range. The glutaraldehyde and atrazine showed little effect on the surface charge of the membranes. However, atrazine in the presence of humic acid showed a significant effect on the zeta potential, causing the membrane to have a less negative surface charge than in the presence of the humic acid alone.