Concurrent measurements of gas-phase hydrogen peroxide and ozone, along with meteorological parameters, were made from July 1 to July 28, 1995, at Whiteface Mountain, a rural, high-elevation site in the northeastern United States. H2O2 and O3 exhibited variability from day to day, with concentrations ranging from 0.15 to 5.15 parts per billion by volume (ppbv) (mean, 1.61¿0.90 ppbv) and from 29 to 108 ppbv (mean, 50.2¿14.6 ppbv), respectively. We present three case studies to elucidate the influence of atmospheric chemical and dynamical processes on the ambient concentrations of H2O2 and O3. Elevated levels of these two photochemical oxidants were observed during July 12--14, owing to an intense photochemical activity in the presence of a synoptic high-pressure system centered over the northeast. Nocturnal maxima in H2O2 and O3 concentrations were observed and are thought to be due to mixing processes injecting air from aloft into the nocturnal boundary layer. While H2O2 showed a weak diurnal variation with a maximum concentration during daytime, O3 showed a weak reverse diurnal variation with nighttime concentrations higher than daytime. A strong positive correlation was observed between H2O2 and O3, suggesting that O3 serves as a source of hydroperoxy radicals (HO2), the precursor of H2O2. The concentration of HO2 in the atmosphere was estimated to be 23 parts per trillion by volume from the observed rate of production of hydrogen peroxide, which is in good agreement with the values reported based on direct measurements and modeling studies.¿ 1997 American Geophysical Union