We have estimated the contributions of mass concentrations of major aerosol species (sulfate, carbonaceous material, and sea salt) to cloud condensation nuclei (CCN) number concentrations from simultaneous measurements of aerosol number size distributions, impactor-derived mass size distributions of aerosol species, and CCN number concentrations (measured at 0.5% supersaturation). Our approach involves the derivation of number size distributions from the impactor data by numerical data inversion from which the mass contributions of sulfate, organic material and sea salt to CCN number concentrations are estimated. The utility of this approach is demonstrated by the results obtained at Point Reyes, California, where we compared the derived number size distributions with directly measured number size distributions. Direct measurements of the CCN concentrations for 0.5% supersaturation showed agreement of ¿25% with CCN derived from impactor data. The contribution of sulfate to CCN concentrations was found to be between 20 and 65% versus contribution of organic particles between 4 and 80%. Thus sometimes sulfate can contribute most of the CCN, while at other times organic aerosols dominate the CCN concentrations. ¿ American Geophysical Union 1995