The rate of incorporation of radiolabeled sulfur dioxide has been determined in submicron sized ammonium sulfate droplet aerosols with and without catalytic metal ions (Fe3+, Mn2+). The sulfate droplets were generated by nebulizing solutions with a multiple jet Collison nebulizer and aged up to 30 min in a 10 m3 plug-flow reaction duct. Radiolabeled 35SO2 was metered into purified air to provide a concentration of 5 ppm. Three different atmospheres were studied: SO2 in purified air, SO2 in the presence of ammonium sulfate aerosol (1 mg m-3, 1 μm MMAD), and SO2 in the presence of ammonium sulfate aerosol containing Fe3+ and Mn2+ ions. No measurable SO2 conversion was detected in samples from atmospheres without the catalytic metal ions. A net SO2 conversion rate equivalent to 0.02 % h-1 was observed in the presence of Fe3+ and Mn2+ ions. © 1985.