During the Indian Ocean Experiment Intensive Field Phase 99, aerosol time-of-flight mass spectrometry (ATOFMS) was used for on-line characterization of the size and chemical composition of individual particles. High temporal resolution single-particle measurements were made on the Island of Kaashidhoo and on board the Research Vessel Ronald H. Brown during February and March, respectively. Sea-salt and dust particles are the major contributors to the aerosol between 1.0 and 2.5 μm. For particles with aerodynamic diameters between 0.2 and 1.0 μm, carbon-containing particles dominated. Differences in the chemical composition of the sampled aerosol were observed for different days and locations, and can be correlated with back trajectory information. For locations north of the Intertropical Convergence Zone (ITCZ), anthropogenic influences on the aerosol chemical composition were indicated by increased contributions from sea-salt particles with non-sea-salt (nss) sulfates, dust with sulfates, and carbon-containing particles with potassium and sulfates. In contrast, in regions close to and south of the ITCZ, the aerosol showed characteristics consistent with a more pristine marine environment. The relative contributions from sulfate-associated particles were evaluated from the single-particle data, as well. The observed sulfate aerosols over the size range analyzed in this study (i.e., aerodynamic diameter between 0.2 and 2.5 μm) consist exclusively of sulfates associated with sea salt, dust, elemental carbon, and/or organic matter. Quantitative contributions from the different types of sulfate-associated particles are evaluated at different locations during the IFP: southern latitude, midlatitude, northern latitude at the Arabian Sea, and mid/northern latitude at the Bay of Bengal. ¿ 2001 American Geophysical Union