The techniques developed to estimate the impact of alternative transportation systems on the air quality of the Sydney Region are discussed. These techniques are centered around the development of a vehicle emissions inventory and a simplified diffusion model of the type proposed by Hanna. The use of these techniques is demonstrated by applying them, in conjunction with emission data on stationary sources, to estimate present pollutant concentrations in Sydney. The results show that both motor vehicles and industry are important emitters of nonmethane hydrocarbons and oxides of nitrogen, the precursors to the formation of oxidants. The enforcement of standards requiring emission reductions similar to the 1973 U.S. motor vehicle standards will go some way toward alleviating Sydney’s oxidant problem. However, more stringent standards will be required to reduce ambient oxidant concentrations to the World Health Organization’s long term goal of 0.06 ppm.