The effect of temperature and O-2 concentration on the formation of NO, N2O, HCN, and NH3 was studied during oxidative regeneration of the spent CoMo/Al2O3 and NiMo/Al2O3 hydroprocessing catalysts. The experiments were performed isothermally in successive steps lasting 6 h each at 350, 450, and 500 degrees C. Helium and 2 and 4% O-2 were used as the media. For both catalysts, the amount of N-containing emissions accounted for about one-third of the total nitrogen in the coke. Most of the N2O, HCN, and NH3 formation occurred in the same temperature range as that of CO and CO2, whereas the NO formation persisted until the very end of every burning step. The amount of coke on the catalyst influenced burning patterns. For the spent NiMo/Al2O3 catalyst, the chemically controlled burn was much more evident than that for the CoMo/Al2O3 catalyst. The deposits of metals such as vanadium and nickel present in the former have contributed to the difference. The availability of O-2 was a much more important factor during burn of the CoMo/Al2O3 catalyst than during that of the NiMo/Al2O3 catalyst.