The maximal reactant conversions in isothermal double-membrane reactors are discussed theoretically, in comparison to single-membrane reactors. A method for calculating the limiting conversion is proposed and demonstrated on the elementary reversible gas-phase reaction aA = bB + cC. The criteria for approaching the limiting conversions are presented, in terms of values of dimensionless model parameters. The complete conversion of reactant can be realized in double-membrane reactors concerning the reactions with low equilibrium conversion and realistic process parameters. As to the applicability of double-membrane reactor model that assumes reaction equilibrium, that approximation could be reasonable only when the maximal attainable conversion is lower than the complete conversion of reactant. To demonstrate the validity of the conclusion that double-membrane configuration is superior to the single-membrane configuration considering the maximal attainable conversion, the results of simulation of thermal water splitting are presented. (C) 2001 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.