Existing kinetic mechanisms were assessed, arriving at the conclusion that a 'better' model was needed. Included are the results of the validation study for one of the more promising existing mechanisms and a discussion of the deficiencies of this and other formulations. A new kinetic mechanism is presented. The results of recent validation studies, which are detailed, demonstrate that this model is capable of predicting with acceptable accuracy the concentration/time behavior of smog chamber experiments for propylene, isobutylene, n-butane, and a mixture of propylene and n-butane at initial NO(x) to hydrocarbon ratios of 1/3 to 1. The mechanism has also been shown to simulate accurately the effect on photo-oxidation rates of variations in CO concentrations, as well as the inhibitory effect of high initial concentrations of nitric oxide on the maximum concentration of ozone obtained. The adaptation of this validated mechanism for use in an urban airshed model is discussed.