Multi-objective optimization of the operation and design of a styrene manufacturing process has been studied with the elitist nondominated sorting genetic algorithm (NSGA-II). In the first part, the study focused on bi-objective optimization and comparative analysis of three different styrene reactor designs-the single-bed, the steam-injected and the double-bed reactors. The objectives were to simultaneously maximize styrene flow rate and styrene selectivity. In the second part, on the other hand, a tri-objective optimization study was performed involving the entire manufacturing process consisting of the reactor, heat-exchangers and separation units. Only the double-bed reactor was considered in this study for maximizing the styrene flow rate and selectivity and minimizing the total heat duty required by the manufacturing process. Results are presented and discussed in detail. (C) 2004 Elsevier Ltd. All rights reserved.