Formaldehyde, acetaldehyde, acetone, and butanone were measured in 59 samples of marine air. Average clean air background concentrations were about 0.55, 0.50, 0.38, and 0.03 ppbv, respectively, in agreement with past measurements. The formaldehyde concentration is also in agreement with that predicted from photooxidation of methane and other locally derived organic matter. Formaldehyde and acetaldehyde showed strong diurnal variations throughout the 12-day sampling period. Photochemical oxidation of locally derived organic matter, such as nonmethane hydrocarbons and long-chained lipids, appears to be the major source for both formaldehyde and acetaldehyde in the lower marine boundary layer. Acetone, on the other hand, showed weaker diurnal fluctuations, consistent with its significantly longer lifetime in the troposphere. Sinks for carbonyl compounds in the lower marine boundary layer are less clearly known. Our results suggest that photolysis, reaction with free radicals, and deposition at the sea surface are minor, short-term sinks during our study. The main sink appears to be vertical mixing, probably followed by photolysis in the upper marine boundary layer and free troposphere. ¿ American Geophysical Union 1993