Carbonyl sulfide (OCS) plays a role in the earth's radiation balance and has ground-based sources that include fossil fuel combustion and emissions from the ocean. Based on spatial sampling, it appears that the coastal ocean is the largest component of the marine source. In order to more accurately quantify the variability of coastal OCS fluxes and to investigate mechanisms that lead to enhanced OCS concentrations in coastal waters, a three-year field and laboratory study of estuarine OCS was undertaken. The Chesaeake Bay was used as a model estuary for the study, and non-contaminating sampling and analytical methods were employed. Results from the water column of the chesapeake Bay show no definitive seasonal trends in the sea-air flux of OCS, although these fluxes are over 50 times those from the open ocean. In contrast, fluxes of OCS from estuarine sediments under different conditions (temperature, carbon substrates, and inhibitors) demonstrate that OCS production is coupled to microbial sulfate reduction. These results explain the high sediment-water fluxes in the summer, and combined with the field data, provide evidence that anoxic sediments are an important component of the OCS budget in the coastal waters.