This report describes an evaluation of the Pollution Episodic Model (PEM) using the St. Louis Regional Air Pollution Study (RAPS) data. This evaluation is designed to test the performance of the model by comparing its concentration estimates to the measured air quality data, using appropriate statistical measures. Twenty days, ten summer and ten winter, are selected from the RAPS data base for the PEM evaluation. The model's performance is judged by comparing the calculated 12-hour average concentrations with the corresponding observed values for five pollutant species, namely, SO2, fine and coarse sulfates, and fine and coarse total mass. A first-order chemical transformation of SO2 to fine sulfate is considered in the calculations in addition to the direct emission and dry deposition of all five pollutants. The model domain, covering 125 x 125 km with a 50 x 50 receptor grid, includes 286 point sources and 36 area sources in the greater St. Louis urban area. Hourly meteorological data and detailed emission inventories for the five pollutants are used as inputs to the model. For the twenty PEM evaluation days, PEM predicted average concentrations of SO2, and fine and coarse sulfates to within a factor of two. The model overpredicted the average concentrations of fine and coarse total mass by a factor of three to four over the evaluation period. This is attributed primarily to overestimation of emission rates and incorrect location of area sources, which dominate the fine and coarse total mass emissions.