The surface charge properties of membrane materials used for water desalination are among the important factors that influence membrane performance. Therefore, the electrical characterization of the membranes is critical for understanding and predicting the filtration capability as well as the fouling potential of the membranes. The zeta potential of three commercial RO membranes was determined for aqueous solutions of various chemical compositions over a pH range of 4 to 11, using a streaming potential analyzer. The isoelectric points for these membranes were estimated and the curves of zeta potential versus solution pH were compared. The effects of salt concentration and ionic charge on the electrical properties were also investigated. Results indicated that while monovalent ions behave in a similar way over the working pH range, divalent cations raise the zeta potential towards more positive values and a higher isoelectric point. On the other hand, divalent anions generally maintain the same zeta potential value but lower the isoelectric point.