Do anthropogenic emissions affect cloud microphysical properties? One parameter needed to answer the preceding question is a link between the aerosol number distribution and the cloud condensation nuclei (CCN) supersaturation spectrum for polluted and unpolluted air masses. On a south-north cruise in the Indian Ocean in March of 1998, we found that measured CCN spectra and CCN spectra calculated from the ambient aerosol number distributions disagreed by as much as a factor of 2. The aerosol soluble fraction inferred from the relationship between measured and calculated CCN spectra ranged from 0.2 to 0.05 and exhibited a broad minimum at the Intertropical Convergence Zone (ITCZ). The concentration of CCN, non-sea-salt SO42-, NH4+, black carbon, and organic carbon increased by an order of magnitude as we passed through the ITCZ. ¿ 2000 American Geophysical Union