"This report deals with the continued development and testing of a general kinetic mechanism for photochemical smog formation. In line with recent experimental measurements, several rate constant values were updated, and simulations of several n-butane/NOx, propylene/NOx, and n-butane/propylene/NOx smog chamber experiments were repeated. The predictions made tend to be best at high ratios of initial hydrocarbons to NOx. A sensitivity analysis of the mechanism was carried out, and the results were combined with uncertainty estimates of the rate constants to quantify the importance of determining individual rate constants with greater accuracy. Operating parameters of the University of California, Riverside, (UCR) evacuable smog chamber were considered in detail; experimental data from this chamber will soon be used to test the mechanism further. Finally, the report discusses the validity of the steady-state approximation in simulating smog chamber experiments and some techniques for mathematically combining a number of similar organic species into general groupings." "EPA 650-4-74-040." "November 1974." Cover title.