During the fall of 1981 and the summer of 1982, airborne and ground-level field studies were conducted in southern and central Ontario to investigate the processes leading to cloud water acidification. Aircraft measurements were made of aerosol particles, cloud droplets, and cloud liquid water content. Cloud water samples were collected in nonprecipitating cumulus and stratocumulus clouds and analyzed for the concentrations of SO4 =, NO3 -, Cl-, Na+, NH4 +, CA++, Mg++, and H+. Aerosol composition was studied at a rural location on the ground. The ground-level aerosol composition was compared with the cloud water composition. Observations of the vertical variation of the aerosol particle size distribution water used to justify this procedure, but it was also assumed that the relative fraction of submicron aerosol SO4= did not vary between the ground and cloud base during individual flights. In this way the cloud water SO4 = was explained by the nucleation of water droplets on sulphate aerosol particles in at least nine of 11 cases studied. In several of the cases the concentration of NO3 - measured in cloud water samples was much higher than could be accounted for by the measurement of below-cloud aerosol NO3 - plus gaseous HNO3. Similarly, concentrations of Ca ++ found in the cloud water far exceeded concentrations measured in the aerosol.