The adsorption and reactions of thiophene (C4H4S) On sulfided Mo/Al2O3 catalysts have been investigated over wide ranges of pressure (10(-9)-5 x 10(2) Torr) and temperature (140-693 K) using infrared (IR) spectroscopy and temperature-programmed desorption (TPD). When dosed at 190 K, thiophene adsorbs molecularly onto sites located on MoS2-like structures and on uncovered alumina regions of sulfided Mo/Al2O3 catalysts as determined by IR spectroscopy. Thiophene is weakly chemisorbed to the catalyst surface and desorbs in a single peak with a maximum rate of desorption at 243 K. Based upon interpretation of IR spectra, the adsorption mode of thiophene on the sulfided Mo portion of the catalyst surface has been determined to be eta(1)(S), with thiophene bonded to coordinately unsaturated (cus) Modelta+ sites located on the edge planes of MoS2-like structures. The saturation coverage of thiophene on MoS2-like structures of a sulfided 17.6 wt % Mo/Al2O3 catalyst is estimated to be C4H4S/Mo = 0.074. Thiophene is observed to become reactive on sulfided Mo/Al2O3 catalysts only in the presence of gas-phase hydrogen and at high temperatures (i.e., 693 K). Temperature-programmed desorption in ultrahigh vacuum following heating of a sulfided 9.4 wt % Mo/ Al2O3 catalyst to 693 K in a thiophene/H-2 mixture results in desorption of C-4 hydrocarbons, H2S, and H-2 from the catalyst surface.