Bulk and size-segregated aerosol samples were collected from the NOAA R/V Ronald H. Brown as it cruised from Cape Town, South Africa, through the Indian Ocean and into the Bay of Bengal and Arabian Sea (February to April 1999; 33¿S to 19¿N). Throughout the Northern Hemisphere, aerosol loading was greater than in the Southern Hemisphere. Samples collected in air that had passed over India showed evidence of fossil fuel combustion, biomass burning, and eolian material, with elemental carbon (EC) dominating radiation absorption and the following relative contributions to the total mass of aerosol particles: ash 29%, nss-sulfate 22%, sea salt 15%, nitrate 9%, organic material 8%, ammonium 6%, and EC 5%. Careful examination of the coarse mode revealed substantial concentrations of nitrate, adequate to acidify sea salt aerosols north of the Intertropical Convergence Zone. Air that had passed over Arabia showed little evidence of biomass burning but had more acidity, mineral dust, and higher nitrate to sulfate ratios than air from India. High concentrations of mineral dust played a major role in radiation absorption; mean contributions to aerosol mass in Arabian air were: ash 38%, nss-sulfate 10%, sea salt 33%, nitrate 5%, organic material 4%, ammonium 1%, and EC 1%. From the ship we measured an average bulk aerosol concentration of 20 ¿g m-3 in the marine boundary layer of the northern Indian Ocean.