In situ oxidation of SO2 has been determined in clouds as a function of droplet size using a trace element technique during July 1998 at Whiteface Mountain, New York. The pH of the cloud water ranged from 2.8 to 4.7 with a mean of 3.4, and therefore SO2 oxidation was dominated by hydrogen peroxide. Size-fractioned cloud samples were collected from six different events at the mountain's summit (1.5 km above mean sea level) using a size-fractionating California Institute of Technology Active Strand Cloudwater Collector. Bulk samples were collected using both passive and active collectors. During each event, below-cloud and interstitial aerosols were collected every 2 hours. Cloud water and aerosol samples were analyzed for major ions and selected trace elements. Continuous measurements of gas phase species SO2, H2O2, and O3 were carried out at the summit and below-cloud sites. Concentrations of cloud water SO42-, NO3-, H2O2, and trace elements, as well as pH, were largely independent of droplet size. The component of cloud water SO42- produced from in situ oxidation (SO4in2-) was also largely independent of droplet size. The results are in agreement with calculated relative production rates in the small and large drop sizes based on known laboratory reaction rates. ¿ 2001 American Geophysical Union