Carbon monoxide (CO) was rapidly formed when rainwater samples were exposed to sunlight or light from a solar simulator. The initial photoformation rate of CO for rainwater samples collected in Miami, FL ranged from 22 nM/h to 200 nM/h at summer clear-day-noon time. The initial CO formation rate was also strongly correlated with the concentrations of dissolved organic carbon in rainwaters. This correlation indicates that dissolved organic matter is the major substrate for the photochemical production of CO in atmospheric liquids. Carbonyl compounds are suggested to be an important species in the photochemical production of CO in atmospheric waters. When assuming the global average concentration of dissolved organic carbon (DOC) of 2.0 mg liter-1, the global photoproduction rate of CO in atmospheric liquid-phase is estimated to be of the order of 200--300 Tg per year, which is about 10% of the total global CO emissions from all sources. This newly found source of CO can significantly influence the redox chemistry in the atmosphere, particularly in cloud droplets and in the surrounding air. ¿ American Geophysical Union 1996