Atmospheric acidification is the result of the oxidation of sulfur, nitrogen, and organic compounds to form their corresponding acids. The gas and aqueous-phase pathways depend on the production of oxidizing free radicals (HO, CH3O2) that react directly with these compounds or produce molecular oxidants. The most important molecular oxidants are H2O2, organic peroxides, and O3. Except for O3-olefin reactions, these molecular oxidants are not reactive in the gas phase, but they are highly reactive in the aqueous phase with dissolved reductants (SO2, HNO2, organics). Thus, the molecular oxidants may be generated in a photochemically active region and transported long distances before reacting with dissolved reductants in the aqueous phase, such as fogs, clouds, and rain. Changes in H2SO4 and HNO3 atmospheric production rates are expected to be a function of changes in the emission rates of SO2, NOx, and VOC. A reduction in emission rate of SO2 will lead to a reduction in gas-phase production of H2SO4. However, if SO2 concentration exceeds that of H2O2 reduction in SO2 emissions may not lead to a reduction in aqueous-phase production of H2SO4. Effective control of H2O2 production is dependent on scientific advances to determine its formation mechanisms and rates.