The adsorption of thiophene (C4H4S) on almost stoichiometric TiO2(110) and defective TiO2-x(110) surfaces has been studied with a combination of synchrotron-based high-resolution photoemission spectroscopy, thermal desorption mass spectrometry (TDS), and first-principles density-functional (DF) slab calculations. The bonding nature between C4H4S and Ti or O sites of TiO2(110), and point defects (oxygen vacancies) of TiO2-x(110) was investigated. Over an almost stoichiometric TiO2(110) surface, the adsorption and desorption of C4H4S is completely reversible. In the submonolayer regime, four adsorption states in the temperature range of 150-450 K were identified in TDS. No thiophene decomposition was observed up to 800 K under ultra-high vacuum (UHV) conditions. The results of DF calculations indicate that at small coverage the molecule should be bonded with its ring nearly parallel to the surface. At high coverages of thiophene, bonding through the S atom becomes more stable because it reduces adsorbate <----> adsorbate repulsion. On a defective TiO2-x(110) surface with similar to45% of Ti3+ and T2+, a small fraction of the adsorbed thiophene molecules (<0.05 ML) decomposed. Our experimental and theoretical studies indicate that the vacancy states of TiO2-x(110) interact poorly with the LUMO of thiophene. This is a behavior opposite to that found for the adsorption of S-2, CH3S and SO2. The role of Ti3+ sites in the desulfurization activity of MoSx/TiO2 catalysts is analyzed in light of these results. (C) 2003 Elsevier Science B.V. All rights reserved.