A new generalized kinetic mechanism for photochemical smog, which incorporates recent information on the atmospheric reactions of aromatic hydrocarbons, has been developed. The mechanism, labeled the Carbon-Bond Mechanism III (CBM III), is the third lumped-parameter mechanism to be designed in accordance with the carbon-bond reaction concept in which carbon atoms with similar bonding are treated similarly, regardless of the molecules in which they occur. Because of the general nature of the CBM III, it can be used to model the entire atmospheric mix of hydrocarbons and is suitable for use in air quality simulation models. Principal features of CBM III include a separate reaction scheme for ethylene, realistic photochemistry for aromatic hydrocarbons and dicarbonyl compounds, and formation pathways for alkyl nitrates and nitroaromatic compounds. CBM III was tested by comparing the predictions obtained with the mechanism against smog chamber data of multi-component hydrocarbon/NOx mixtures. In addition to a discussion of the development and testing of the CBM III, information is also provided on the application of the mechanism for urban air quality modeling.