In the water treatment industry, the use of artificial intelligence is well established, particularly in deep well pumping or wastewater treatment. Drinking water treatment plants, however, have made little use of expert systems based on artificial intelligence. This paper describes an expert system designed for a water treatment plant using ozone. The expert system is a tool to help treatment plant designers and operators predict the performance of ozone contactors or micropollutant removal. The author's purpose was to create an expert system that could be used by both designers and operators for three objectives: disinfection, micropollutant removal, and reduction of optical density. Two case studies are offered, one of a simulation of an existing plant and the other using the expert system to design a new ozonation unit.