Carbonyl sulfide (COS) was measured in surface seawater and in marine air during two Atlantic cruises of the R/V Polarstern between Bremerhaven, Germany, and Cape Town, South Africa. The cruises took place in the fall of 1997 and in the summer of 1998. The concentration of COS showed clear diurnal, seasonal, and latitudinal variations, as did its saturation ratio. The concentration of dissolved COS averaged 14.7 pmol L-1 and 18.1 pmol L-1 for the fall and summer cruises, respectively. On most days, seawater was undersaturated in COS during the late night and early morning but was supersaturated during the rest of the day, implying that the ocean can act as both a source and a sink for COS on the same day. The COS content in seawater was correlated significantly with the global radiation, the CH3SH concentration, and the seawater temperature. The air-sea flux of COS from the open Atlantic Ocean was estimated using exchange coefficients calculated according to Erickson's stability dependent model for air-sea gas exchange. The largest COS flux into the atmosphere occurred in productive regions (the Benguela Current, the West African upwelling area, and the northeastern Atlantic) during the warmer seasons. A small net oceanic uptake of COS was found in the Benguela Current during the southern winter. The average open ocean fluxes were 13.5 nmol COS m-2 d-1 and 28.6 nmol COS m-2 d-1 for the two cruises, respectively. A global open ocean source of 0.10 Tg COS yr-1 is extrapolated from the measured data. The atmospheric mixing ratio of COS averaged 474¿33 and 502¿38 pptv for the fall and summer cruises, respectively, and had no significant interhemispheric gradient. ¿ 2001 American Geophysical Union