Pretreatment of a new lanthanum molybdenum sulfide (LaMoS) hydrodesulfurization (HSD) catalyst in flowing H-2 has been investigated. Following pretreatment up to 800 degrees C, the catalyst surface area decreased from 175 to 11 m(2)/g; the average pore diameter increased due to the loss of pore diameters smaller than 100 Angstrom. TPD characterization of the as-prepared LaMoS was used to monitor the evolution of solvents and adsorbed materials up to 800 degrees C. The Mo oxidation state was examined after various pretreatment procedures using X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (TPR). Following temperature-programmed desorption (TPD), two types of surface oxygen were detected which could be reduced at 250 degrees C and 950 degrees C by a flow of 10% H-2 in N-2. Transformation of the amorphous LaMoS to the corresponding crystalline Chevrel phase [La(Mo6S8)] was detected at 1000 degrees C using TPR and x-ray diffraction (XRD).