The attainable region (AR) method is used to obtain the optimal design and operating conditions for the adiabatic oxidation of sulfur dioxide to sulfur trioxide. The optimum process layout for this exothermic, catalytic, reversible reaction is a complex interconnection of reactors that achieves a conversion in excess of the equilibrium value for a single adiabatic reactor. The complex design obtained from the AR method is technically and economically superior to a single adiabatic reactor but it is inferior from a safety perspective as a result of its complex interconnections and its high preheat temperature. The strategies of inherently safer design (minimize, substitute, moderate, and simplify) have been applied to the complex design to compare it with conventional adiabatic reactor designs.