Hydroperoxide measurements are presented for 12 flights of the U.S. Department of Energy G-1 aircraft during the summer 1995 intensive of the Southern Oxidants/Middle Tennessee study. A three-channel analyzer, utilizing both peroxidase/p-hydroxy phenylacetic acid (pOHPAA) and ferrous sulfate/benzoic acid (FeBA) reagents permitted continuous measurements of hydrogen peroxide (H2O2), methyl hydroperoxide (CH3OOH or MHP), and hydroxymethyl hydroperoxide (HOCH2OOH or HMHP). The median concentration of total hydroperoxide was 5.2 ppbv, with median concentrations of 2.4, 1.7, and 0.97 ppbv for H2O2, MHP, and HMHP respectively. In the free troposphere, H2O2 concentration correlated with the concentration of its precursors, ozone, and water vapor. H2O2 profiles do not show elevated concentrations in the boundary layer, in contrast to vertical profiles of primary pollutants. The combination of high NOx concentrations that inhibited peroxide formation, and peroxide loss through deposition and reaction with OH, led to markedly lower peroxide concentrations within urban and power plant plumes. Photochemical calculations performed to estimate the magnitude of in-plume peroxide depression showed reasonable agreement with observations. ¿ 1998 American Geophysical Union