Two catalysts containing 2 wt.% of ruthenium or palladium on alumina (CRu and CPd, respectively) were prepared by impregnation from chloride precursors and then calcined and reduced at 573 K. The catalysts were characterized by gas chemisorption, TPR and FT-IR using CO as probe molecule. The catalytic activity was evaluated in the o-xylene hydrogenation at 5 x 10(5) Pa, 77.6 h(-1) and 363-393 K, in a fixed-bed tubular reactor and the sulfur resistance analyzed at 373 K with different feeds contaminated with 20, 50 and 100 ppm of thiophene, respectively. CRu is more active and selective toward cis-1,2-dimethylcyclohexane (c-DMCH) than CPd, but it is less sulfur resistant. The selectivity to 1,2-dimethylcyclohexane isomers (S-t and S-c) for each catalyst is scarcely influenced by the reaction temperature, but it is affected by both the presence of thiophene in the liquid feed and the degree of deactivation of the catalysts by sulfur. CO/FT-IR measurements of the fresh catalysts showed that, in addition to Pd-0, an important part of palladium in CPd is present as Pddelta+ while Ru-0 is by far the major species in CRu. The IR spectra of the deactivated samples suggest the presence of thiophene adsorbed on the surface, which is removed outgassing at 573 K and 10(-3) Pa. After this treatment the deactivated samples do not completely recover either the metal surface or the capability to chemisorb CO. The catalytic performances are related to the nature of the metal, the ratio of the adsorption coefficients of thiophene and o-xylene (K-th/(xil)) and the electronic state of the active metal sites, which are modified by effect of thiophene. The decrease of the S-t/S-c ratio along the run suggests a parallel decrease of the surface electron deficiency produced by the chemisorbed thiophene. (C) 2003 Elsevier Science B.V. All rights reserved.