In the last few years, a renewed interest in the water gas shift (WGS) reaction at low temperature has arisen due to its application to fuel cells. In this work, a simulation of a fixed bed reactor for this reaction, which forms part of a hydrogen production-purification train for a 10 kW PEM fuel cell using ethanol as the raw material, was carried out. A commercial Cu/Zn/Ba/Al2O3 catalyst was employed and a one-dimensional heterogeneous model was applied for the simulation. The catalyst deactivation due to thermal factors (sintering) was taken into account in the model. Isothermal and adiabatic regimes were analyzed as well. Results of the simulation indicate that the pellet can be considered isothermal but temperature gradients in the film cannot be disregarded. On the other hand, concentration gradients in the film can be ignored but CO profiles are established inside the pellet. Adiabatic operation can be recommended because of its simplicity of operation and construction. The reactor volume is strongly sensitive to the CO outlet concentration at CO levels lower than 6000 ppm. For a 10 kW PEM fuel cell, using adequate pellet size and taking into account the catalyst deactivation, a reactor volume of 0.641 would be enough to obtain an outlet CO concentration of about 7160 ppm. This concentration value can be handled by the next purification stage, COPROX. (c) 2005 Elsevier B.V. All rights reserved.