The characteristics of Ru/TiO2 Catalysts Sulfided at temperatures from 573 to 873 K were studied by TPRS, XRD, electron microscopy, EPR, and UV-Visible-NIR. It was found that the structure of the ruthenium sulfided phase formed at the different sulfidation temperatures is essentially the same when supported on alumina or titania. However, when supported on titania effects arising from electronic transfers between the Ru species and the TiO2 support affect the performance of the catalyst. The high HDS activity of Ru/TiO2 sulfided at 573 K was explained through the electronic interaction between metallic Ru (coming from the partial reduction of RuS2) and titania. The low HDS activity of Ru/TiO2 sulfided at 873 K was explained by the alteration of the already optimum electronic configuration of ruthenium sulfide caused by electron density supplied by the titania support. This provides experimental evidence to support the reported theoretical correlations between electronic structure and HDS activity. (C) 2009 Elsevier Inc. All rights reserved.