Hydrogen peroxide, organic hydroperoxide species, and formaldehyde were measured in the troposphere over the South Atlantic, Brazil, and southern Africa during the NASA Global Tropospheric Experiment (GTE) Transport and Atmospheric Chemistry Near the Equator-Atlantic (TRACE-A) experiment, an investigation into the cause of an observed seasonal ozone maximum in this region. H2O2 and CH2O concentrations were extremely high, up to 14 and 5 ppbv, respectively, and are the result of direct production in fire plumes as well as secondary photochemical production in the atmosphere. Enhanced hydroperoxide and formaldehyde concentrations have a significant effect in both odd-hydrogen and odd-oxygen cycles in the troposphere. The odd-hydrogen radical production from hydroperoxide and formaldehyde photolysis exceeded that from ozone photolysis at high altitudes. At low altitudes where air was impacted by biomass burning the contribution of these species to odd-hydrogen radical production was comparable to that of O(1D). The fraction of radical production due to H2O2 and CH2O photolysis linearly increased with their concentrations. Odd-hydrogen radical production from hydroperoxides is ~30% of the total. These species also react with OH and cycle odd-hydrogen from OH to HO2 and CH3O2 radicals. In and near biomass burning plumes, more hydroperoxide and formaldehyde reacted with OH than was photolyzed, and HO2 radical production from H2O2 and CH2O oxidation was also important. Hydroperoxide and formaldehyde from fires together with nitric oxide initiate odd-oxygen production and contribute to new O3 formation. The greatest destruction of odd-oxygen in continental outflows leads to additional photochemical enhancement of the hydroperoxide. Therefore enhanced hydroperoxide and formaldehyde from biomass burning increases the oxidizing capacity of the atmosphere near fires and their high concentrations downwind reflect the stimulated chemistry. ¿ 1998 American Geophysical Union