Methane sulfonic acid (MSA) is an oxidation product of the reaction of OH radical with dimethyl sulfide and, hence, should be important constituent of marine air, MSA concentrations in marine aerosols ranged from 0.009 to 0.075 μg/m3 in samples from the Pacific and Indian oceans and Miami, Florida. In the samples from remote areas (Pacific and Indian oceans). MSA levels averaged 6.7% (S=1.9) of the non-sea-salt (nss) SO4= values. In the Miami area, ratios were occasionally lower because of the impact of local sulfur emissions (probably pollutant SO2). MSA concentrations in seven rainwater samples collected at Miami, Florida, ranged from 0.001 to 0.034 ppm. Cascade impactor samples from Miami, Florida, and the Gulf of Mexico indicate that MSA occurs primarily in the smaller particles, as does nss SO4=. In the two samples analyzed, MSA is concentrated on slightly coarser particles than nss SO4=. The size distributions of both MSA and nss SO4= differ markedly from that of sea salt Na+, suggesting that at least the first step in the oxidation of organosulfur compounds is a gas phase reaction. The observed concentrations of MSA in marine air are the result of formation from gas phase reaction of dimehtyl sulfide (and other volatile organosulfur compounds) with OH radical and destruction by aqueous phase reaction with OH radical in aerosols. Preliminary experiments indicate that MSA oxidation probably yields SO4= under atmospheric conditions. Therefore, MSA destruction is a potentially important pathway for the formation of nss SO4= in the marine atmosphere. The magnitude of the fluxes involved in the organosulfur cycle cannot be calculated from the MSA data because of the uncertainty in the free radical chemistry of aerosols.