Based on the data presented in this study, microorganisms can degrade wet cellulose acetate (CA) reverse osmosis (RO) membranes during shutdown periods or under storage conditions. Fungal (and perhaps bacterial) metabolism of the membranes was measured from CA membranes using the C-14 isotope. Using uniformly labeled C-14 cellulose as the starting compound, CA polymer was synthesized, casting solutions were made, and CA membranes were produced. After inoculating membrane samples with the microorganisms, the metabolism of the membranes was confirmed by measuring isotopically enriched carbon dioxide gas produced either by the respiration under aerobic or anaerobic conditions using the Krebs metabolic cycle or by fermentation. Results show that the dense zone of the asymmetric membrane, where desalting occurs, could be compromised in 3 months or less. Membrane samples that were treated by sanding and hydrolyzing the surface to simulate a "used" membrane, degraded at a faster rate than new membranes, Removing oxygen from the system failed to slow or stop the rate of growth. Data showed that no "special" fungus from the RO industry is unique in degrading CA membrane. Instead, all organisms tested degraded these membranes, including microorganisms found in the biofilms from a field RO system and microorganisms present as laboratory contaminates.