We present a way to minimize the entropy production rate in plug flow reactors using optimal control theory, a well known branch of optimization theory. A general formulation is first presented for a plug flow reactor with several reactions, when the temperature of the cooling/heating medium is the control variable. The Hamiltonian of the optimal control problem is constant. We present some solutions of the optimization problem for SO2 oxidation. It is shown that reductions in the entropy production rate up to 25% can be achieved by varying the reactor length and controlling the utility. Furthermore, the results shed light on properties of the optimal states. An hypothesis is proposed that a reactor with high energy efficiency has relatively long sections with almost constant driving forces and entropy production rate. (C) 2004 Elsevier Ltd. All rights reserved.