The in-cloud reaction between SO2 and H2O2 has been investigated using data from field campaigns of July 1995 and July-August 1997 at Whiteface Mountain, New York. Cloud water samples were collected at the mountain's summit (1.5 km, above mean sea level (amsl)), and aerosols in clear air at a site (Lodge) located at 0.6 km amsl and in cloud interstitial air at the summit. Cloud water and aerosol samples were analyzed for SO42- and selected trace elements. Gaseous SO2 and H2O2 were measured at both sampling sites. Criteria based on total sulfur and O3 concentrations were developed to decide when the two sites were coupled (i.e., air masses at the two sites had the same composition for the chemical species studied). Four case studies are presented that exhibit varying concentration levels and different meterological conditions. In two cases the sites were coupled, and in two others they were decoupled. The clouds showed high pollutant concentrations with a mean pH of 3.5. A tracer technique was used to quantitatively determine the SO42- produced from in situ SO2 oxidation. In general, good agreement between the SO2 consumed and SO42- produced was observed. Similarly, H2O2 concentrations in precloud air showed stoichiometric mass balance with the sum of H2O2 observed in cloud interstitial air, in cloud water, and equimolar amounts needed to produce SO4in2- as determined by the tracer technique. Data from all 24 clouds showed that (1) ~28% of the cloud water SO42- was due to in situ oxidation; (2) ~75% of the SO2 in the precloud air was oxidized in situ at Whiteface Mountain; (3) the mean SO2 concentration in air masses associated with cloudy periods is approximately 1.4 ppb, significantly higher than the summer average of 0.5 ppb; (4) the mean H2O2 concentration in cloud-free periods during summer is 1.1 ppb sufficient to oxidize SO2 present; (5) the frequent presence of H2O2 in cloud water indicated that the reaction had not often reached completion. ¿ 2000 American Geophysical Union