The main purpose of the current contract effort is the development and validation of a simulation model for estimating ground level concentrations of photochemical pollutants. A major component of the model is a generalized kinetic mechanism capable of describing atmospheric chemical reactions developed by Prof. John H. Seinfeld at the California Institute of Technology. The authors wished to modify the treatment of nitrous acid in the mechanism, to include additional categories of lumped hydrocarbons and their reactions, to improve means for estimating the generalized stoichiometric coefficients, and to assess more fully the mechanism's validity. It was the purpose of this effort to carry out validation of the mechanism, as modified, for a variety of reactive systems, including single hydrocarbons, binary mixtures, and auto exhaust. The mechanism provides an adequate, perhaps good, description of smog chamber kinetics.