In this study the performances of a membrane reactor (MR) are estimated when both shell side stream (sweep gas) and lumen side stream are continuously either in parallel flow configuration (co-current mode) or in counter-flow configuration (counter-current mode). Two mathematical models have been formulated and steady-state mass-balance gave two-dimensional differential equations, which were solved by using the orthogonal collocation technique. Simulation results for both co-current mode and counter-current mode have been compared in terms of hydrogen molar fraction (in the shell side) vs. axial co-ordinate at different hydrogen permeances, temperatures, and lumen pressures. At the operative conditions considered, a very similar CO conversion value has been obtained for both modes. (C) 2003 Elsevier B.V. All rights reserved.