The production of high-purity hydrogen using the water gas-shift reaction in both conventional fixed bed reactor and hydrogen perm-selective membrane reactor at low to medium scale is studied in this work by developing and comparing models with different complexity levels. A two-dimensional rigorous reactor model considering radial and axial variations of properties (including bed porosity), setting mass, energy and momentum differential balances, and nesting a rigorous model for mass transfer within the porous catalyst was considered as reference for comparison. Different simplifications of this model for taking into account mass-transfer effects within the catalyst pellet (Thiele modulus, evaluation of apparent kinetic constants, empirical correlation for effectiveness factors or just neglecting these effects) were tested, being observed that these effects are not negligible and that the first two approaches are accurate enough for taking into account mass transfer within catalyst pellets. Regarding to the reactor model, it was observed that one-dimensional models are not adequate, especially for the membrane reactor. Analogously, neglecting the momentum balances in the reactor (as made is most simulations reported in the literature) leads to important misspredictions in the behaviour of the membrane reactor performance. Finally, the influence of the main operation parameters (inlet temperature, pressure, space velocity, etc.) was studied using the detailed reactor model, concluding that space velocity and pressure are the most important parameters affecting reactor performance for membrane reactors. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.