The maximal reactant conversions in cocurrent isothermal double-membrane reactors are discussed theoretically, in comparison to single-membrane reactors. A method for calculation of the limiting conversion is proposed and demonstrated on the elementary reversible gas phase reaction A = B + C. Owing to the disturbing separation and reaction equilibrium, double-membrane reactor without sweep gas could provide complete conversion. When the sweep gas is introduced, large but not infinite sweep gas flow enables complete conversion. The criteria for approaching the limiting conversions are presented, in terms of values of dimensionless model parameters. 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. 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 unity.