An Eulerian grid-based photochemical airshed model was used to simulate the formation of ozone and other photochemical compounds in the Metropolitan Area of São Paulo. The first step was the development of a base case scenario with good agreement levels between measured and simulated concentrations. In all simulations, only mobile sources, which are considered responsible for the majority of the emissions of ozone precursors, were considered. The comparison between the measured and observed concentration showed good agreement mainly for CO and NO, indicating an adequate representation of vehicular emission profile for light and heavy-duty fleet. Accurately estimating emissions from the motor vehicle fleet of an urban area was one of the most problematic steps in the entire emissions inventory process. The uncertainty in this case is mainly related to the low level of accuracy of the local emissions inventory, which also contributed to the discrepancies observed when comparing the simulated and the experimental ozone concentration profiles. This is an abstract of a paper presented at the 98th AWMA Annual Conference and Exhibition (Minneapolis, MN 6/21-24/2005).