Distributions of organic hydroperoxides (ROOH), hydrogen peroxide (H2O2), other trace gases, and meteorological parameters were measured from an aircraft over the northeastern United States in August and September 1988. The data set consists of 13 flight missions on 14 days with 177 vertical profiles at altitudes between 100 and 3500 m. The mixing ratio of ROOH varied from below the detection limit of 0.1 parts per billion by volume (ppbv) up to 3.14 ppbv, with generally higher average values in the free troposphere (FT) than in the planetary boundary layer (PBL). The average ratio of ROOH to the H2O2 mixing ratios was about 50%. The ROOH mixing ratio was strongly correlated with H2O2, indicating similar production and loss mechanisms for ROOH and H2O2. The observed lower variability of the ROOH mixing ratios in comparison with that of H2O2 is probably due to a longer atmospheric residence time of ROOH. Combined with the lower ROOH mixing ratios, this longer residence time indicates that the production and loss processes of ROOH are slower than those of H2O2. An ROOH formation rate of 56¿8 parts per trillion by volume ROOH/h% H2O was determined from the difference of ROOH mixing ratios between afternoon and morning flights on September 16 . This formation rate is about twice as large as the rate predicted by a simple photochemical box model with methane chemistry, indicating that organic hydroperoxides other than CH3OOH may also be formed. The estimations of HO2 and RO2 concentrations in the free troposphere from H2O2, ROOH, and O3 formation rates were consistent.