During the AEROSOLS99 cruise over the Atlantic and Southern Indian Oceans, peroxy radicals (RO2*=HO2+&Sgr;RO2, where R is an organic group) were continuously measured. Regular diurnal variations were observed with maximum mixing ratios of 30 to 55 parts per trillion by volume around noon. The latitudinal distribution of RO2* over the ocean seems to remain uniform under similar conditions of solar insolation. The composition of the marine boundary layer reflects long-range transport of air from the African continent. The influence of biomass burning over remote areas of the marine boundary layer is confirmed, although the atmosphere over the Southern Hemisphere was often pristine, with rapid destruction of ozone observed during the daylight hours. Observed RO2* concentrations are in reasonable agreement with box model calculations, but cannot account for the highest diurnal variations of ozone observed. Possible explanations include halogen chemistry and nocturnal entrainment of O3 from the layers above, but available data cannot readily determine their relative contributions. ¿ 2001 American Geophysical Union