In situ measurements of aerosol optical and hygroscopic properties were made over the Indian Ocean onboard the National Center for Atmospheric Research (NCAR) C-130 aircraft as part of the 1999 Intensive Field Phase (IFP) of the Indian Ocean Experiment (INDOEX). Research flights were conducted primarily in the Northern Hemisphere to the west and southwest (i.e., downwind) of the Indian subcontinent, although several flights crossed the Intertropical Convergence Zone (ITCZ) into the much cleaner Southern Hemisphere air. The optical and hygroscopic properties of submicrometer aerosols were measured most of the time, although measurements on aerosol particles smaller than ~3 ¿m have also been reported. Low-altitude (0--1 km altitude) measurements of the submicrometer aerosol light scattering coefficient (σsp, adjusted to standard temperature and pressure) in the INDOEX pollution aerosol showed a median value of 53 Mm-1, which is a factor of ~2--5 higher than the median values during polluted periods at remote North American marine/coastal sites but similar to those observed at rural U.S. stations during high-aerosol periods. Submicrometer light absorption coefficients were even higher compared with the North American measurements, at 4--23 times those median values. Single-scattering albedo (ω0) measurements showed that the Indian Ocean pollution aerosol was highly absorbing, with mean values at ambient relative humidity between 0.84 and 0.87 for low-altitude flight segments conducted in the Northern Hemisphere. The aerosol hygroscopic growth factor, defined as f(RH) = σsp (RH=85%)/σsp (RH=40%), averaged 1.58 for flight segments below 1 km altitude and north of 5¿N. This is a substantially lower f(RH) than typically observed at midcontinent Northern Hemisphere sites, although it is nearly identical to that observed at these sites when aerosols were influenced by agricultural burning or dust episodes in the surrounding area. Aerosol optical properties over the Indian Ocean also showed significant variability in both the horizontal and the vertical on very short spatial and temporal scales. Cloud-free level flight segments of only ~10--30-min duration showed a mean variability (standard deviation/mean) in the 1-min average ambient aerosol extinction coefficient of ~18% and a mean difference between the lowest and the highest segment extinction measurement of 39%. Vertical profiles conducted over the Northern Hemisphere Indian Ocean showed two major types of profiles. One type showed nearly constant to decreasing aerosol scattering with increasing altitude, while the other type displayed elevated aerosol layers that did not appear to be associated with boundary layer aerosols and were occasionally separated from them by relatively clear layers. These intense elevated layers, which were present in 52% of all profiles, often showed scattering coefficients several times as large as those measured near the surface. A change in synoptic-scale circulation patterns halfway through the project may have caused more of these elevated decoupled layers to be observed during the second half of the IFP and may have caused them to be observed at higher altitudes.