Technical Abstract: Chlorpyrifos is the most widely used insecticide in the Chesapeake Bay region. Recent studies show that this organophospate chemical is consistently present in the air, rain and surface waters of the Chesapeake Bay region, suggesting a long environmental half-life. Hydrolytic degradation of chlorpyrifos is likely a dominant removal process, but existing hydrolysis data does not reflect conditions in the Chesapeake Bay In this project, hydrolysis rates of chlorpyrifos were measured in sterilized, ambient water from the mouth of four Chesapeake Bay tributaries ranging in salinity from 0-17 ppT. The measured hydrolysis half-lives varied from 24-d in the Patuxent River to 126-d in the Susquehanna River. Copper concentration emerged as an independent predictor from a multiple variable correlation analysis of the water constituent data. Results show that even though total copper concentration in these natural waters was very low, the catalytic effect significantly shortened the hydrolysis half life of chlorpyrifos over base or neutral-catalyzed hydrolysis alone.