The mass transfer in microchannel reactors with catalyst-coated walls is investigated. A limitation by internal mass transfer can be tested by the variation of the temperature or the thickness of the catalyst layer. Due to the laminar flow in the microchannels the external mass transfer relies solely on the molecular diffusion. Suitable test methods for the external mass transfer are: variation of temperature, channel diameter at constant residence time, total pressure at constant partial pressures of the reactants, or change of the inert gas. For the selective oxidation of isoprene in a microchannel reactor mass transfer limitations are experimentally excluded by the variation of catalyst particle diameter and the change of the inert gas nitrogen to helium. Using a two-dimensional model the radial profiles of the reactants in the microchannels are simulated. Two different criteria to distinguish between the kinetically and the mass transfer controlled regime are presented. (c) 2005 Elsevier B.V. All rights reserved.