Continuous measurements of ozone (O3) and hydrogen peroxide (H2O2), and of other trace gases and meteorological parameters, were made using the NOAA King Air research aircraft. The atmosphere was sampled at various elevations from minimum altitude above ground level up to 4 km above sea level, over the northeastern United States between Columbus, Ohio, and Saranac Lake, New York, during June 1987. The average O3 concentrations, calculated for constant-altitude flight segments, are in the range 50--110 ppbv; the H2O2 range for the same flight segments is 0.6--3.6 ppbv. Concentration profiles of O3 show little variation with altitude, with the exception of samples taken in polluted areas. The highest H2O2 concentrations are observed at the lowest altitudes in air from southern regions, i.e., in air containing the greatest water vapor density. The concentrations of both O3 and H2O2 show a clear decrease from lower (40 ¿N) to higher (44 ¿N) sampling latitudes. ¿ American Geophysical Union 1990