Stream muds and sediments, contaminated with arsenic from an industrial waste water, have been studied, under laboratory conditions, for release of this contaminant to overlying waters. The arsenic content of the muds was determined initially whereupon simulated reservoirs were prepared from these muds and natural stream water. The systems studied were: anaerobic conditions at 25o, 20o, 15°, 10°, and 4°C, aerobic conditions at 20°and 10°C with dissolved oxygen (DO) levels of 100%, 50%, 25%, and 10% saturation. Distribution coefficients, the dimensionless ratio of AS /AS ,, were determined. In both the water mud aerobic and anaerboic systems, lower temperatures produced smaller distribution coefficients (those in the 4–10oC range were about 40% of the coefficients at the higher 20–25°C range). Very little difference was found in the distribution coefficients at a given temperature between reservoirs having either 10%, 25%, 50%, or 100% of saturation of dissolved oxygen. Anaerobic conditions led to levels of arsenic, principally as As (+III), about ten times higher than concentrations reached with aerobic conditions. The ratio of anaerobic As level/aerobic As level in reservoir waters was decreased also with decreasing temperature (8.3 at 25°C, and 5.6 at 10°C). The arsenic in the aerobic reservoirs was found to be about 70% arsenate, and 20% organic‐arsenic. Little pH effect was observed over a pH range of 6.0 to 8.5.