A review is provided about techniques that have been used to evaluate the effective diffusivity of gases in catalyst/washcoat layers, as used in catalytic monoliths.The importance of making such measurements is described, in order to ensure that the choice of model for effective diffusivity can be verified, and if necessary an appropriate value of tortuosity can be back-calculated. Based on methods described in the literature, it is concluded that, where possible experiments should be performed on actual monolith structures, rather than those that have been reformed. The chromatographic technique is applied to a catalytic monolith and preliminary results of unpublished work are presented. A method of using a cut section from a catalytic monolith in a modified form of 'Wicke-Kallenbach diffusion cell' is also described.Examples from the patent literature are provided showing, how interest in layered catalyst systems has started to grow, illustrating how diffusion in porous layers can be exploited to develop 'designer catalyst systems'. (C) 2003 Elsevier B.V. All rights reserved.