The classical theory of diffusion and reaction inside catalytic pellets, based on the effectiveness factor concept, is applied to thin coatings supported on microchannels or nonporous pellets. We take advantage of this distinct geometrical characteristic to simplify the general problem. In particular, we show that the contribution from the geometry curvature can be written in terms of the effectiveness factor for a slab catalyst. An approximate method for the evaluation of eta is proposed, which is accurate for linear and nonlinear kinetics, in uniform and nonuniform geometries. This presents a more convenient procedure for calculating eta in structured reactors for process intensification. (C) 2011 Elsevier Ltd. All rights reserved.