Our laboratory studies indicate that the photochemical production of carbonyl sulfide (COS) in seawater can result from the photosensitized reaction of organosulfur compounds. Because natural photosensitizers are more prevalent in coastal waters than the open ocean, these results help to explain why marine COS concentrations are highest in coastal and shelf regions. Wavelength studies of COS photoproduction in freshly collected seawater samples demonstrated that COS formation is induced primarily by the UV part of solar radiation. Studies with model organosulfur compounds in synthetic and coastal seawater indicated that compounds with bivalent sulfur atoms (thiols, sulfides) react most efficiently to produce COS. We propose that the initial step of the photosensitized reaction primarily involves attack on the sulfur atom or sulfhydryl H atom by reactive transients. ¿ American Geophysical Union 1994