On-line capabilities of modem expert systems are of special interest due to the possibility of real-time environmental monitoring and detection of possible rules violations in their early phase of development. One of the most difficult environmental problem for some refineries located in fairly populated areas are released refinery odors. In an attempt to solve this problem, an odor expert system is proposed in this paper. It includes a historical data base with all available odor complaints, real-time monitoring of weather data and selected process variables, a detailed map, and rules. The rules are based on the developed methodology for quantitavely correlating process variables with odor complaints. The methodology includes threshold values calculations for selected variables at the Waste Water Treatment Plant and their correlation with process variables in different units inside the refinery. The proposed expert system has been implemented in the GENSYM G2 software environment at StarEnterprise refinery in Delaware City, Delaware, USA.