The promotional effects of Sn on Pd/SiO2 have been investigated by the comparison of Pd/SiO2 and Pd-Sn/SiO2 activity in the hydrogenation of maleic anhydride and succinic anhydride to gamma-butyrolactone, and XPS measurement. When reduced at 200 degreesC, both catalysts showed a strong dependence on the reactants, and the addition of tin led to an increase of gamma-butyrolactone yield. When reduction temperature was increased to 350 degreesC, however, the influence of the reactants in the activity disappeared in the case of Pd-Sn/SiO2. The disappearance of the feed effect may be suggested to be due to a decrease of the adsorption strength of maleic anhydride on active sites induced the formation of bimetallic Pd-Sn sites. A decrease of the deactivation rate found on Pd-Sn/SiO2 can also be explained by the decreased rate of polymerization due to the diminution of the adsorption strength of maleic anhydride on Pd-Sn sites. As a result, it is proposed that Pd-Sn/SiO2 catalyst can be an effective one-step catalyst for hydrogenation of maleic anhydride to gamma-butyrolactone. (C) 2002 Elsevier Science B.V. All rights reserved.