Atmospheric particles acidify through the processes of nucleation, condensation, coagulation, and gas-particle chemical reactions. The gas-phase pathway depends on the generation of free radicals that oxidize sulfur dioxide, nitrogen dioxide, and organic compounds to acids, followed by either nucleation or condensation. The aqueous-phase pathway acidification proceeds through the reactions of dissolved reductants and oxidants. The pathways for accumulation of sulfuric acid, nitric acid, and organic acids in particles may differ in responses to factors such as liquid water content of the particles and mass accommodation coefficients of reactants. Laboratory chemical kinetic results indicate that photochemical gas-phase and co-absorbed oxidant reaction pathways for S(IV) may be important simultaneously, for example, in industrial-urban air during the summer. However, during the night and during the winter, aqueous-phase pathways should dominate. Laboratory investigations of the oxidation of nitrogen dioxide indicate that the gas-phase photochemical pathway is important, but the aqueous-phase is not. Gas-phase photochemical oxidation of organics is generally established, but the aqueous-pathways have not received sufficient attention to establish their importance.