Chlorine chemistry has been incorporated into the carbon bond IV mechanism and employed in a regional photochemical model (the Comprehensive Air Quality Model with Extensions (CAMx)) for preliminary use in assessing the regional impact of chlorine on ozone formation in Houston, Texas. Mechanisms employed in regional photochemical models do not currently account for chlorine chemistry. However, when chlorine chemistry is accounted for, predicted ozone levels are enhanced by up to 16 ppbv in the Houston area, with the greatest enhancement predicted for morning hours after sunrise. Thirteen reactions have been added to the chemical mechanism used by CAMx to describe chlorine chemistry in the urban atmosphere. The reactions include photolysis of chlorine radical (Cl¿) precursors, Cl¿+ hydrocarbon reactions, and Cl¿+ ozone reactions. The hydrocarbon reactions include the reactions of Cl¿ with isoprene and 1,3-butadiene that yield unique reaction products, or marker species. The development of this mechanism is presented along with a discussion of the initial set of predictions of chlorine-based ozone enhancement in the Houston area. Of significant interest is that methane may be activated by chlorine to contribute significantly to the predicted ozone enhancement in the Houston area. Such behavior suggests that the impact of chlorine chemistry would be proportional to the availability of Cl¿ precursor. In urban areas with anthropogenic sources of chlorine radical precursors, chlorine radical chemistry may be important to more accurately predict ozone formation.