The performance of TiO2-modified Pd catalysts, containing TiO2 either as an additive or as a support, in the selective hydrogenation of acetylene was investigated using a steady-state reaction test. The catalyst surface was characterized by H-2 and CO chemisorption, infrared, X-ray photoelectron spectroscopy (XPS), and the temperature-programmed desorption (TPD) of ethylene. The TiO2-added I'd catalyst reduced at 500degreesC, denoted in this study Pd-Ti/SiO2/500degreesC, showed a higher selectivity for ethylene production than either the Pd/TiO2 or Pd/SiO2 catalyst. The amounts of chemisorbed H-2 and CO were significantly reduced and, in particular, the adsorption of multiply coordinated CO species was suppressed on Pd-Ti/SiO2/500degreesC, which is characteristic of the well-known strong-metal-support-interaction (SMSI) phenomenon that has been observed with the TiO2-supported Pd catalyst reduced at 500degreesC, Pd/TiO2/500degreesC. XPS analyses of Pd-Ti/SiO2/500degreesC suggested an electronic modification of Pd by TiO2, and the TPD of ethylene from the catalyst showed the weakening in ethylene adsorption on the Pd surface. 1,3-Butadiene was produced in smaller amounts when using Pd-Ti/SiO2/500degreesC than when using Pd/SiO2/500degreesC, indicating that the polymerization of C-2 species leading to catalyst deactivation proceeds at slower rates on the former catalyst than on the latter. The enhanced ethylene selectivity on Pd-Ti/SiO2/500degreesC may be explained by correlating the catalyst surface properties with the mechanism of acetylene hydrogenation.