Temperature-programmed oxidation tests were carried out on catalysts which were aged in a bench-scale unit, using either a vacuum gas oil (VGO) feed or a Kuwait atmospheric feed. The VGO-aged catalysts displayed one CO2 peak from oxidation of coke and two SO2 peaks from oxidation of inorganic sulfur on the catalyst. The temperature at which the CO2 peak appeared increased with the coke content. For the Kuwait-aged catalysts, a second, high-temperature carbon oxidation peak was observed when moderate amounts of vanadium were present. The carbon oxidation peaks were closely accompanied by SO2 peaks. The coke in the latter catalysts essentially covered the inorganic sulfur phase as well as adsorbed nitrogen compounds. It was found that the aged catalysts could lose a considerable amount of sulfur during the extraction procedure if too high a temperature was used during post-drying.