In a recent paper (J. Catal, 158, 411, 1996) we demonstrated that the intensity of a specific peak (pK similar to 6) in proton affinity distributions (PADs) measured for a series of Co-Mo/Al2O3 catalysts could be correlated to the variation in HDS activity within the series. PADs access information about species on the catalyst’s surface from analyses of proton binding data collected using potentiometric titration. If the species determined at the solid/aqueous interface are present after catalyst activation and are active during catalytic testing, then this methodology can provide a simple, surface sensitive procedure for catalyst characterization which should correlate with other characterization techniques. To test this hypothesis we provide herein results from more conventional catalyst characterization techniques in order to corroborate the PAD results. The techniques of temperature-programmed reduction, X-ray photoelectron spectroscopy, and laser Raman spectroscopy were used to provide additional characterization of Co-Mo/Al2O3 catalysts in their oxide state. PADs were obtained for this catalyst system in their oxide and sulfided states, The HDS activity for each member of this series was measured using hydrodesulfurization of thiophene as a test reaction. The effect of cobalt loading, molybdenum content, presulfidation conditions, and pH during catalyst preparation were the variables used, Based on the experimental data we propose that the active site for HDS activity is a Co-Mo interaction species consisting of molybdate octahedra and a dispersed Co which could also be octahedrally coordinated. This species can be transformed reversibly during the processes of sulfidation and reoxidation and its formation is controlled by pH.