Controlled cloud chamber experiments were conducted to measure particle growth resulting from the oxidation of SO2 by O3 and H2O2 in cloud droplets formed on sulfuric acid seed aerosol. Clouds were formed in a 590 m3 environmental chamber with total liquid water contents ranging from 0.3--0.6 g m-3 and reactant gas concentrations <10 ppbv for SO2 and H2O2 and <70 ppbv for O3. Aerosol growth was measured by comparison of differential mobility analyzer size distributions before and after each 3--4 min cloud cycle. Predictions of aerosol growth were then made with a full microphysical cloud model used to simulate each individual experimental cloud cycle. Model results of the H2O2 oxidation experiments best fit the experimental data using the third-order rate constant of Maass et al. <1999> (k=9.1¿107 M-2 s-1), with relative aerosol growth agreeing within 3% of measured values, while the rate of Hoffmann and Calvert <1985> produced agreement within 4--9%, and the rate of Martin and Damschen <1981> only within 13--18%. Simulation results of aerosol growth during the O3 oxidation experiments were 60--80% less than the measured values, confirming previous results <Hoppel et al., 1994b>. Experimental results and analyses presented here show that the SO2-O3 rate constants would have to be more than 5 times larger than currently accepted values to explain the measured growth. However, unmeasured NH3 contamination present in trace amounts (<0.2 ppb) could explain the disagreement, but this is speculative and the source of this discrepancy is still unknown. ¿ 2001 American Geophysical Union