A simple 'box-approach' to air quality simulation modeling has been developed in conjunction with a newly formulated photochemical kinetic mechanism to produce an easily applied Photochemical Box Model (PBM). This approach represents an urban area as a single cell 20 km in both length and width and with a variable height representing the changing depth of the mixed layer. Each pollutant species is accounted for in the model by an ordinary differential equation composed of advective volume expansion, chemical, and emissions (if any) terms. Initial development and testing of the PBM has drawn upon the Regional Air Pollution Study (RAPS) data base, a comprehensive set of meteorological and air quality data for St. Louis, Mo. One-minute averaged solar radiation data are used in the computation of the photolytic rate constants for the model. Hour-averaged winds and air quality data are utilized in initial and boundary condition calculations as well as providing the observed values for use in model validation. The upper and lower bounds on the mixed layer depth are deduced from RAPS rawinsonde data. Finally, model predictions from the PBM are examined and compared to the observed data on selected days. Application of the model for prediction of oxidant levels in urban areas is discussed.