Hydrogen peroxide, formaldehyde, and a measure of organic hydroperoxides are presented from the Mauna Loa Observatory Photochemistry Experiment (MLOPEX). MLOPEX was conducted from May 1 to June 4, 1988, at the Mauna Loa Observatory on the island of Hawaii. A modified dual-enzyme serial-coil H2O2/ROOH method was used to quantify these species. A second enzyme method was used to measure CH2O. The location and meteorology at the Mauna Loa Observatory site permitted 35 days of measurements to be made in free tropospheric air and in modified marine boundary layer air. Average concentrations of H2O2, ROOH, and CH2O were 1050, 140, and 100 pptv in free tropospheric, or down-slope air. In upslope air, or modified marine boundary layer air, average concentrations were 900, 150, and 190 pptv. Maximum concentrations for all three species were experience during a two day ''photochemical haze'' episode and were 3230, 440, and 450 pptv for H2O2, ROOH, and CH2O. H2O2 was depleted in air which had recently been processed by cloud or precipitation. The measured concentrations of these three species were comparable to prior measurements in ''well-aged'' air, but were lower than previous models have predicted. Part of this discrepancy may be due to the treatment of heterogeneous removal processes, dry and wet deposition, in these models. The measured ratio of ROOH to H2O2 is significantly different than present theory predicts, with ROOH as measured being approximately a factor of 5 too low. ¿ American Geophysical Union 1992