Abstract. Measurements of cloud condensation nuclei (CCN) were made in downtown Toronto during August and September, 2003. CCN measurements were performed at 0.58% supersaturation using a thermal-gradient diffusion chamber, whereas the aerosol size distribution and composition were simultaneously measured with a TSI SMPS and APS system and an Aerodyne Aerosol Mass Spectrometer (AMS), respectively. Aerosol composition data shows that the particles were predominately organic in nature, in particular for those with a vacuum aerodynamic diameter of <0.25 µm. In this study, the largest contribution to CCN concentrations came from this size range, suggesting that the CCN are also organic-rich. Using the size and composition information, detailed CCN closure analyses were performed. In the first analysis, the particles were assumed to be internally mixed, the organic fraction was assumed to be insoluble, and the inorganic fraction was assumed to be ammonium sulfate. The AMS time-of-flight data were used for Köhler theory predictions for each particle size and composition to obtain the dry diameter required for activation. By so doing, this closure analysis yielded an average value of CCN_predicted/CCN_observed=1.12±0.05. However, several sample days showed distinct bimodal distributions, and a closure analysis was performed after decoupling the two particle modes. This analysis yielded an average value of CCN_predicted/CCN_observed=1.03±0.05. A sensitivity analysis was also performed to determine the aerosol/CCN closure if the organic solubility, droplet surface tension, or chamber supersaturation were varied.