This paper reports the results of a numerical investigation of the diffusion and reaction in the washcoat of a catalytic monolith reactor. Both idealized shapes and real geometries are tested. It is shown that for non-uniform washcoat thickness, the Sherwood number varies along the channel/washcoat interface. The internal diffusion resistance, expressed in terms of an effectiveness factor, cannot be represented in terms of a unique curve using the generalized Thiele modulus approach. The most significant deviation occurs when the washcoat has the greatest variation in thickness. The internal diffusion affects the magnitude of the external mass transfer resistance. Three factors significantly affect the rates and role of mass transfer. These are (1) the washcoat thickness, (2) the channel radius, including its non-uniformity around the channel, and (3) the angular diffusion in the washcoat caused by variable thickness. (C) 2004 Elsevier Ltd. All rights reserved.