The effect of desiccation/oxidation on the potential for phosphorus release from reservoir sediments under anoxic conditions was studied. When sediments were exposed to the air and subsequently dried, their ability to release P under anaerobic conditions was severely limited relative to sediments that were not exposed. Anaerobic P release in response to the addition of a toxicant (formaldehyde), bioavailable carbon sources (acetate or glucose), available sulfur (sulfate), and sulfide were used to infer possible causes for the decrease in anaerobic P release as a function of desiccation/oxidation. A substantial reduction in P release occurred in sediments that had not been exposed when formaldehyde was added. This suggests that anaerobic P release may be microbially driven. Addition of acetate and sulfate slightly enhanced P release from nondesiccated sediments, suggesting some C and(or) S limitation of the bacterial processes occurring in these sediments. Substantial P was released when sulfide was added to the nonexposed sediments, indicating that there is a significant amount of P associated with sulfide-reducible minerals (e.g. iron oxyhydroxides). Sulfide addition to exposed sediments released only about one-fourth of the P compared with nonexposed sediments. Furthermore, no additional P was released when sulfate was added to exposed sediments, but P was released in the presence of either glucose or acetate. Together these results indicate that the decrease in potential for P release from exposed sediments was caused by a shift in bacterial community structure (specifically the loss of viable sulfate-reducing bacteria), carbon limitation as a result of drying, and aging of minerals with which P is associated.