The analysis and results are presented of a program of validation and sensitivity analysis of the steady-state Gaussian plume type of urban diffusion model. A definition is developed of the fundamental short-term steady-state model and its various modes of implementation, in terms of emission and environmental input parameters, and of calculational modes. A set of computer programs developed especially for validation and sensitivity study purposes is described. Comparisons were made of short-term and long-term concentration predictions from the model, with comparable measured SO2 concentrations covering three months of two-hour values at ten locations in St. Louis, and one month of one-hour values at eight locations in Chicago. Hourly estimates of meteorological and emission parameters were used.