Submicron aerosol particles (Dp < 1 ¿m) were sampled with stacked filter units on the National Center for Atmospheric Research (NCAR) Hercules C-130 aircraft during February--March 1999 as a contribution to the Indian Ocean Experiment (INDOEX). We determined the vertical and spatial distribution of the major aerosol components (NH4+, Na+, K+, Mg2+, Ca2+, methyl sulfonic acid, Cl-, NO3-, SO42-, oxalate, organic carbon, and black carbon) over the Indian Ocean to examine the role of pollution aerosols on indirect and direct radiative forcing. High pollution levels were observed over the entire northern Indian Ocean down to the Intertropical Convergence Zone (ITCZ) located between the equator and 10¿S. In the northern part of the Indian Ocean (5¿--15¿N, 66¿--73¿E), high concentrations of carbonaceous aerosol and pollution-derived inorganic species were found in a layer extending from the sea surface to about 3.5 km asl. In this layer, the average mass concentration of all aerosol species detected by our technique ranged between 7 and 34 ¿g m-3, comparable to pollution levels observed in industrialized regions. In the Southern Hemisphere (1¿--9¿S, 66¿--73¿E), the aerosol concentrations rapidly declined to remote background levels of about 2 ¿g m-3. The concentrations of non-sea-salt sulfate (the main light scattering component) ranged from maximum values of 12.7 ¿g m-3 in the Northern Hemisphere to 0.2 ¿g m-3 in the Southern Hemisphere. Carbonaceous aerosol contributes between 40% and 60% to the fine aerosol mass of all determined components. An unusually high fraction of black carbon (up to 16% in the polluted areas) is responsible for its high light absorption coefficient.