The steam reforming of methane in a membrane reactor has been simulated with emphasis on the effect of hydrogen removal by the membrane on the poisoning effects due to traces of H2S. The performance of the membrane reactor under these conditions has been compared with that of fixed-bed reactor operation for temperatures in the range 773-873 K. Removal of hydrogen increases the tendency to poisoning from H2S. This effect can be reduced by operating at higher temperatures when H2S poisoning is dominant. (C) 2000 Elsevier Science B.V. All rights reserved.