Abstract An industrial CoMo/Al 2O 3 hydrotreating catalyst has been subjected to regeneration temperatures between 300 and 700°C. Catalytic activities for hydrodesulfurization (HDS) of thiophene and hydrogenation (HYD) of cyclohexene were largely influenced by these pretreatments. Infrared spectroscopy of the NO-probe, sulfur analyses and X-ray photoelectron measurements were used to interpret the changes brought about during regeneration. Attention has been focused mainly on the merit of the infrared spectroscopy of the NO chemisorption as a surface specific probe for characterizing the surface sites, viz. coordinately unsatured sites, as well as to measure the relative appearance of cobalt and molybdenum atoms in catalysts subjected to increasing regeneration temperatures. At temperature above 500°C the sulfur content, octahedrally coordinated Co species, and HDS (or HYD) activities markedly decreased, while Mo δ+ species increased. A reasonable correlation between the normalized intensities of the infrared bands of NO chemisorbed on sulfided Co 2+ sites (symmetric stretching at about 1850 cm −1) and HDS (or HYD) activities was found.