Special water quality regulations have been proposed to limit lethal acute concentrations of toxic pollutants to a spatially restricted toxic dilution zone. Predictive mathematical models are used to establish the initial dilution of a given discharge and the characteristics of its mixing zone. To assist the analyst in choosing the appropriate models, determining the limits of applicability, and establishing data needs, an expert system has been developed. The structured computer program uses knowledge and inference procedures obtained from experts. The personal-computer-operated program, called CORMIX1, is a subsystem of the Cornell Mixing Zone Expert System. In current testing, CORMIX1 appears to be a highly flexible tool for regulatory analysis that is adaptable to the evaluation of alternatives in engineering design. The knowledge base and model logic are easily understood and thus offer an excellent learning environment.