Pretreatment may constitute up to one-fourth of the total costs of a membrane desalting facility. By using preexisting conventional filtration plants for pretreatment significant cost savings may be realized. However, coagulant residuals from the pretreatment process may negatively affect reverse osmosis (RO) membrane performance. Various RO membranes were tested at three different treatment plants in southern California, using either aluminum sulfate (alum) or ferric chloride coagulants and chloramines. Repeated testing using alum with multiple RO elements revealed rapid deterioration in specific flux (up to 60% over 100 h of operation), as well as progressive reductions in salt rejection (typically 3-4% over 500 h of operation). Microscopic analysis of the fouled membranes revealed that the foulants were primarily aluminum hydroxide and aluminum silicate materials. In contrast to the RO data for alum coagulation, which showed declining membrane flux, the specific flux data using ferric chloride and chloramines increased over time for all membranes. Salt rejection decreased significantly during testing of each membrane. These data suggest that the RO membranes were physically degrading over time. The RO membranes may have been degraded by residual iron catalyzing a chlorine-amide reaction on the membrane surface, despite the fact that chlorine was present as chloramines.