Sulfur dioxide and dimethyl sulfide were determined in the marine boundary layer of the northeast Pacific Ocean west of Seattle, Washington. The mean DMS and SO2 concentrations were 75 and 28 pptv, respectively. During periods of high DMS levels (180 pptv) we found thta SO2 levels remained low (25 pptv) and statistically the same as periods of low DMS. Sulfur dioxide showed no observable diurnal variation indicating that nonphotochemically driven losses to aerosol and other surfaces cannot explain the low SO2 levels observed. We conclude that a low efficiency of conversion of DMS to SO2 is the most likely explanation for the low SO2 levels. Implications of the low yield of SO2 in terms of the kinetics of oxidation of DMS are discussed.