As part of the Indian Ocean Experiment (INDOEX) 1999 Intensive Field Phase, measurements of aerosol properties were made on board the R/V Ronald H. Brown in the Indian Ocean north and south of the Intertropical Convergence Zone (ITCZ) in the Arabian Sea and in the Bay of Bengal. On the basis of air mass trajectories, eight air mass source regions were identified including the southern hemisphere Atlantic; southern hemisphere Indian Ocean; northern hemisphere Indian Ocean; east Indian subcontinent where trajectories came from near Calcutta, through the southern portion of India, and then to the ship; Indian subcontinent where trajectories came from across central India to the ship; Arabia; Arabia/Indian subcontinent, a mixed region where lower-level trajectories came from Arabia and upper-level trajectories came from India; and Arabian Sea/coastal India where trajectories came from along the coast of India to the ship. Properties of the aerosol measured in the marine boundary layer included chemical composition, number size distribution, and scattering and absorption coefficients. In addition, vertical profiles of aerosol backscatter and optical depth were measured. Presented here as a function of air mass source region are the concentrations and mass fractions of the dominant aerosol chemical components, the fraction of the extinction measured at the surface due to each component, mass extinction efficiencies of the individual components, aerosol scattering and absorption coefficients, single scattering albedo, Ångström exponent, and optical depth. All results except aerosol optical depth are reported at the measurement relative humidity of 55 ± 5%. For air masses that originated from the two southern hemisphere marine regions (southern hemisphere Atlantic and Indian Ocean), sea salt dominated the extinction by sub-1 μm and sub-10 μm aerosol particles. The ratios of sub-1μm to sub-10 μm extinction were the lowest measured of all air mass source regions (mean values of 28 and 40%) due to the dominance of the aerosol mass by supermicron sea salt. In addition, aerosol optical depths were the lowest measured averaging 0.06 ± 0.03. Non-sea-salt (nss) sulfate aerosol concentrations in air masses from the northern hemisphere Indian Ocean were a factor of 6 higher than those in southern hemisphere air masses, while submicron sea-salt concentrations were comparable. Sulfate aerosol made up 40% of the sub-1μm extinction, while sea salt dominated the sub-10 μm extinction. Aerosol optical depths for this source region averaged 0.10 ± 0.03. A mean single scattering albedo near 0.89 and detectable black carbon (BC) concentrations (0.14 ± 0.05 μg m−3) indicated the transport of continentally derived aerosol to the ITCZ. The two regions influenced by low-level (500 m) airflow from Arabia had higher concentrations of submicron nss sulfate, particulate organic matter (POM), and inorganic oxidized material (IOM) than were observed in the marine regions. Concentrations of supermicron IOM were comparable to supermicron sea-salt concentrations. Nss sulfate aerosol dominated the sub-1 μm extinction and made significant contributions to the sub-10 μm extinction. Sea salt dominated the supermicron extinction. Mean BC contributions to submicron extinction were 8 and 12%. Single scattering albedo values averaged 0.93 ± 0.02 and 0.89 ± 0.02 for these two source regions. Aerosol optical depths averaged 0.19 ± 0.12 and 0.38 ± 0.07 with the higher value due to upper-level (2500 m) flow from India. Regions influenced by low-level airflow from the Indian subcontinent had the highest submicron nss sulfate, POM, BC, and IOM concentrations measured during the experiment. Supermicron sea-salt concentrations were lower than or comparable to supermicron nitrate concentrations. Sub-1 μm and sub-10 μm extinction were dominated by nss sulfate aerosol although a burning component consisting of BC, KNO3, and K2SO4 made a nearly equivalent contribution. These regions had a mean single scattering albedo of 0.85 ± 0.01, the lowest measured for any region. Mean aerosol optical depths were highest (0.3 to 0.4) for regions with low-level or upper-level airflow from the Indian subcontinent.