An algorithm of diffusive gas transport in porous solids based on random collisions of molecules (DSMC) is extended to include basic heterogeneous reaction mechanisms (adsorption, coadsorption, desorption, and reaction of gas species on the surface of the solid). With this model, we study the catalytic oxidation of CO inside highly porous nanoparticle layers in the transition regime using kinetic parameters from Pd(111) surfaces at ultra high vacuum conditions. Investigation of the reaction at different temperatures reveals a clear transition between a kinetic limit (low temperatures) and a diffusion limit (high temperatures). At high temperatures and under steady-state conditions, the porous layer shows three distinct regions with different reaction rates (reactor poisoning, an effective reaction region, and a region with CO depletion), whose extends are determined by CO concentration and mass-transport limitation. We expect that similar investigations help optimizing the structure of gas sensor elements based on nanoparticle layers fabricated with flame spray pyrolysis. (c) 2015 American Institute of Chemical Engineers AIChE J, 61: 2092-2103, 2015