The FCS (Foulant Component Study) has successfully quantified the effects of different classes of potential membrane fouling materials in the RO (reverse osmosis) feed water at the Yuma Desalting Test Facility upon the decline rates of RO product flow. Fouling components were postulated to be colloidal clays, nonpurgeable dissolved organics, and purgeable halogenated organics. The purpose of the study was to identify the causes for higher than predicted rates of decline in product water flow of two manufacturers' proof test units. Each proof unit contained spiral-wound cellulose acetate membrane elements contracted for the Yuma Desalting Plant. THe RO feedwater was pretreated by chlorination, partial lime softening-clarification, and dual-media filtration. The FCS was conducted using five pairs of 64-mm-diameter RO elements. One element pair served as a control and operated on the proof-test reject water, and four separate pairs were operated after different additional pretreatment steps consisting of ultrafiltration, aeration stripping, activated carbon, and reverse osmosis, each of which removed a different combination of potential foulant components from the RO feed-water. The experiment was done three times, once with one manufacturer's elements and twice with elements from the other manufacturer. Despite similar overall product flow decline rates in the proof tests, the two manufacturers' RO elements responded differently to the FCS pretreatment steps. The product flow decline of one of the manufacturer's elements was virtually eliminated by either ultrafiltration or reverse osmosis pretreatment but not significantly affected by aeration stripping pretreatment, which indicates that colloidal fouling was the overriding cause of the performance decline. In contrast, the performance decline of the other manufacturer's equipment was reduced 60 percent by operation on RO product water, 20 percent by aeration stripping pretreatment, and 20 percent by ultrafiltration pretreatment. For this manufacturer, 20 percent of the performance decline was presumed to be due to membrane compaction. These FCS results indicate that it may be possible to prevent up to 85 percent of the product flow decline rates experienced during proof testing through pretreatment improvements at the Yuma Desalting Plant.