Pd/TiO2 catalysts have been prepared using TiO2 supports consisting of various rutile/anatase crystalline phase compositions. Increasing percentages of rutile phase in the TiO2 resulted in a decrease in Brunauer-EmmettTeller surface areas, fewer Ti3+ sites, and lower Pd dispersion. While acetylene conversions were found to be merely dependent on Pd dispersion, ethylene selectivity appeared to be strongly affected by the presence of Ti3+ in the TiO2 samples. When TiO2 samples with 0-44% rutile were used, high ethylene selectivities (58-93%) were obtained whereas ethylene losses occurred for those supported on TiO2 with 85% or 100% rutile phase. X-ray photoelectron spectroscopy and electron spin resonance experiments revealed that a significant amount of Ti3+ existed in the TiO2 samples composed of 0-44% rutile. The presence of Ti3+ in contact with Pd can probably lower the adsorption strength of ethylene resulting in an ethylene gain. Among the five catalysts used in this study, the results for Pd/TiO2-R44 suggest an optimum anatase/rutile composition of the TiO2 used to obtain high selectivity of ethylene in selective acetylene hydrogenation.