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As described by the Köhler theory, the hygroscopicity of atmospheric aerosol particles is a key factor regulating their cloud condensation nuclei (CCN) activity. Here we investigated the relationship between hygroscopicity and CCN activity for urban aerosol particles using a hygroscopicity tandem differential mobility analyzer (HTDMA) coupled in series to a CCN counter. The HTDMA-CCNC system was operated near the center of the Tokyo metropolitan area from 10 to 17 November 2004. For aerosol particles whose dry mobility diameters were 30–200 nm, the ratios of CCN to condensation nuclei (CN) at 0.22–1.3% supersaturation were obtained as a function of particle hygroscopicity at 83 and 89% RH. More-hygroscopic particles were found to be more CCN active than less-hygroscopic particles of the same size, indicating that hygroscopicity is a critical factor regulating CCN activity. The chemical compositions of particles were simultaneously measured using an aerosol mass spectrometer. They were found to relate closely to CCN activity as well as to the hygroscopicity. The measured CCN–hygroscopicity relationships were compared to those predicted by Köhler theory. The results suggest that CCN activity is possibly perturbed by changes in surface tension due to organics, dissolution/dissociation of water-soluble organics under supersaturation conditions, or different nonideality of organics from inorganic salts. These factors associated with organics are potentially important for CCN activity and thus microphysical cloud processes in the atmosphere.