Measurements of H2O2 and related species were made by aircraft on 18 flights over 11 days during June 1987 over Ohio. Measurements typically encompassed an altitude range from surface to 5.5 km above mean sea level and were conducted under a variety of meteorological conditions, but focussed on conditions conducive to the formation of convective storms. Concentrations of H2O2 ranged from <0.2 to 7 ppb and exhibited as much as a factor of 10 variability even on a given day at fixed altitude, as well as between days. Hydrogen peroxide concentration typically was low near the surface, rose to a maximum near the top of the boundary layer, then slowly decreased with altitude. In contrast, SO2 and NOy were largely confined to the surface layer. A simple photochemical calculation shows that where NOx concentrations are low, 2O2> is strongly influenced by the water vapor concentration. The 2O2> tracked changes in dew point, and measured and calculated H2O2 concentrations were in general agreement. The low 2O2> in the boundary layer is ascribed to consumption of radicals by reactions other than combination reactions, and to loss of H2O2 by dry deposition. Comparison of the profile shapes for H2O2 and SO2 suggests that mixing processes will have a large influence on the extent to which these two species will react in convective systems. ¿ American Geophysical Union 1990