In this study, we examine how emissions from the Indian subcontinent modify the microphysical properties of trade wind cumulus clouds over the Indian Ocean. In situ microphysical data from 18 National Center for Atmospheric Research C130 research flights during the Indian Ocean Experiment (INDOEX) in February-March 1999, in polluted to pristine regions, as delineated from the concentrations of condensation nuclei (CN), were analyzed. Cloud properties were found to vary in a systematic way during the five gradient flights: i.e., those flights in which the C130 flew from about 4.5 ¿N latitude in the high-CN regions, south across the intertropical convergence zone (ITCZ) into the clean regime, then farther south to about 8 ¿S latitude. The high-CN regime contained relatively large concentrations of small droplets as compared to the low-CN regime, where low concentrations of large droplets, and more frequent drizzle, were measured. An analysis of the data from penetrations into thousands of clouds during all 18 flights supports these qualitative observations: In the polluted regions, the droplet concentrations were a factor of 3 higher, the droplet effective diameters were 35% smaller, and drizzle was observed 25% as often than in the pristine regions; in both polluted and pristine regions, the bulk cloud properties (liquid water content (LWC), vertical velocity, cloud horizontal dimensions) were approximately the same. Even larger differences in the microphysical properties between the high-CN, intermediate-CN, and low-CN regimes were noted when the data set was partitioned by the LWC. A high ratio offorward scattering spectrometer probe (FSSP) to CN concentrations was noted in the low-CN regime, whereas in the high-CN regime this ratio was small. Droplet growth calculations in an adiabatic, 1D parcel model over a 300 m cloud depth support the droplet observations and indicate that the cloud optical depths in the high CN regime could have been as large as twice those in the low CN regime as a result of differences in the cloud condensation nucleus (CCN) population. The corresponding albedos of about 0.54 in the high CN regime and 0.47 in the low CN regime signify potentially large differences in albedo for the typical vertical velocities observed in INDOEX clouds, but it is unclear how entrainment would affect this difference. The calculations indicate that stronger updrafts associated with deep convection could lead to a larger difference between the microphysics of high-CN and low-CN regions. ¿ 2001 American Geophysical Union