Ignoring possible effects due to intraphase diffusion within catalyst layers is a common feature of computational fluid dynamics models developed for reforming microchannel reactors. Resistance to diffusion within the catalyst layers applied to such a reactor is often ignored on the grounds that the catalyst layers are sufficiently thin to allow reactants unrestricted access to all available reaction sites. However, this assumption is not necessarily correct, and intraphase diffusion effects could be important. Three-dimensional numerical simulations were carried out using computational fluid dynamics to investigate the characteristics of intraphase transport processes within the catalyst layers arranged in a thermally integrated methanol reforming microchannel reactor. The heat and mass transfer effects involved in the reforming process were evaluated, and the optimum thickness of catalyst layers was determined for the reactor. Particular focus was placed on how to optimize the thickness of catalyst layers in order to operate the reactor more efficiently. The results indicated that the performance of the reactor can be greatly improved by means of proper design of catalyst layer thickness to enhance heat and mass transfer into the catalyst layers. The thickness of the catalyst layers can be optimized to minimize diffusional resistance while maximizing methanol conversion and hydrogen yield. Thick catalyst layers offer higher reactor performance, whereas thin catalyst layers improve catalyst utilization and thermal uniformity. The thickness scale at which intraphase diffusion effects become noticeable was finally determined on the basis of reactor performance. The critical thickness was found to be about 0.10 mm, and catalyst layers should be designed beyond this dimension to achieve the desired level of conversion. The critical thickness will vary depending upon layer properties and operating conditions. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.