The purpose of this research was to determine the accuracy of four photochemical air quality simulation models using data from the Regional Air Pollution Study in St. Louis. The models evaluated in this report are: The Photochemical Box Model (PBM) built in-house by EPA, The Lagrangian Photochemical Model (LPM) built by Environmental Research and Technology, Inc., The Urban Airshed Model (UAM) built by Systems Applications, Inc., and The Livermore Regional Air Quality Model (LIRAQ) built by Lawrence Livermore Laboratory. Emphasis in this report is directed at the ability of the models to reproduce the maximum 1-hour ozone concentrations observed on 10 days selected from nearly two years of data. The PBM, LPM, and UAM have been successfully tested and show potential as air quality management tools. LIRAQ does not show potential as a model for general use, irrespective of its accuracy (which was impossible to judge at this time). The standard deviation of the differences between observed ozone maxima and predicted concentrations at the same place and time tend to be large, ranging 0.04-0.1 ppm for maxima of 0.19-0.26 ppm. Although some problems exist whose resolution could improve model performance, this high variability should be recognized by decision-makers.