Temperature-programmed oxidation (TPO) of a NiMo/alumina hydrotreating catalyst aged with Kuwait resid showed the presence of two distinct types of coke. A low-temperature coke (type I), showing a CO2 peak around 460 degrees C, is attributed to a ’normal’ coke, which is found on catalysts aged with lighter feeds. A high-temperature coke (type II), showing a CO2 peak around 620 degrees C, is associated with vanadium deposits and appears to be consistent with the presence of asphaltenes. The presence of COS in the TPO profiles is taken as evidence of sulfur in the coke. In addition, a low-temperature COS peak is observed, which is ascribed to adsorbed CHx species in the inorganic sulfide phase. The coke effectively covers the inorganic sulfide phase. Hence, appreciable loss of catalytic activity could be due to coverage of active sites by coke, aside from any specific poisoning effects of metal deposits.