Cloud chamber experiments were conducted to measure the rate of conversion of gaseous SO2 to aqueous sulfate under conditions of warm-cloud formation. The rate of SO2 conversion in cloudy air with 140 ppb ozone was approximately 1000 times faster than conversion in clouds without ozone. The rates of SO2 oxidation observed in these experiments with simulated clouds (pH 4.5--5.5) and ozone concentrations in the range of 25--430 ppb are consistent with previously published kinetics for SO2 in bulk-water experiments with much higher concentrations of ozone.