The use of membrane reactors in industrial processes leads to high efficiencies because the reaction equilibrium can be shifted towards high conversion and product formation. In addition, the combination of reaction and separation in a single unit operation leads to process simplification and probably hardware cost reduction. A key design factor for membrane reactors is the ratio of maximum reaction rate per volume over maximum permeation rate per volume, characterised by the product of Damkohler and Peclet numbers (DaPe). The smaller the DaPe number, the more effective the membrane reactor becomes. Using a membrane bed reactor with molecular sieve silica (NISS) membranes and the dehydrogenation of cyclohexane to benzene as the test reaction, we observe that cyclohexane conversion rates increased from 3% to 20% as the DaPe reduced from 80 to 1. The conversion is well predicted by a simple equilibrium model. The DaPe number provides a simple measure of the interaction of the reaction and separation effects and a method to evaluate the membrane reactor efficiency to optimise the design. (c) 2006 Published by Elsevier B.V.