This work provides a numerical model studying mass transport and heterogeneously catalyzed reactions in a porous membrane microreactor. The hydrogenation of nitrite over a Pd catalyst was used as a model reaction. The influence of liquid flow rates, initial nitrite concentration and catalytic membrane layer thickness (wetting thickness) on the conversion was studied. Firstly, a kinetic model was implemented based on the correlations available for reaction kinetics from literature. The results were validated using experimental results and it was found that the process is best described by Langmuir-Hinshelwood reaction kinetics. Secondly, to obtain an optimized reactor geometry, boundary conditions were derived, which represent the reactant concentration at the microreactor inner wall as a function of catalytic layer properties. An optimum in conversion was found for varying catalytic membrane layer thickness. The initial increase in conversion with increasing catalytic layer thickness is due to enhanced catalyst area. The conversion later reduces due to gaseous reactant mass transfer limitation, for even thicker layers. This study provides detailed understanding of the mass transfer taking place in membrane microreactors. It also provides routes towards optimized reactor configurations, which allows for more efficient catalyzed gas-liquid reaction processes. (C) 2012 Elsevier B.V. All rights reserved.