Recent studies of the oxidation of sulfur dioxide by hydrogen peroxide in solution indicate that this process may be the dominant mechanism for the conversion of SO2 to H2SO4 in cloud water where the typical &rgr;H is less than 5.5. In the interpretation of theoretical calculations and previous hydrogen peroxide measurements it has generally been assumed that the limiting factor in the acidification of precipitation is the amount of gas-phase H2O2 available to the cloud-precipitation system. Field observations of H2O2 during Acid Precipitation Experiment (APEX) missions revealed that in addition to vapor phase H2O2, there were present one or more atmospheric constituents which produced H2O2 in the aqueous phase, and there were other species which consumed the collected H2O2 vapor. We report here an investigation of these phenomena undertaken to ensure that the instrumentation was responding to H2O2 vapor. Our results showed that (1) the H2O2 found within the collectors frequently exceeded the amount intially present as vapor, (2) the species which produced H2O2 were relatively insoluble, (3) NO2 did not enhance H2O2 formation, (4) O3 enhanced H2O2 formation, and (5) SO2 depleted aqueous H2O2. The amount of aqueous H2O2 vapor was added to ambient air was larger than expected. The results of these experiments have important implications for cloud water chemistry. If the processes occurring within the impinger can be extrapolated to droplets, the amount of H2O2 available for SO2 oxidation would not be limited by the amount of H2O2 vapor in precloud air but would also depend upon the concentration of the precursors capable of generating H2O2 droplets.