Characterization of coke on equilibrium, fluid catalytic cracking (FCC) catalysts contaminated with metals was investigated using temperature-programmed oxidation (TPO). TPO spectra of spent equilibrium catalysts from cracking of sour imported heavy gas oil (SIHGO) were deconvoluted into four peaks (Peak K, L, M and N). The four peaks were assigned to different types of coke on the catalyst. Peak L in the TPO spectrum was assigned to the 'contaminant' coke in the vicinity of metals. The amount of contaminant coke (Peak L) correlates with metal-contaminant concentration. The size of Peak L which is related to amount of contaminant coke decreased significantly for the spent highly contaminated catalyst pretreated with hydrogen and methane prior to cracking reactions as compared to the non-pretreated catalysts. Since both hydrogen and methane pretreatment can reduce oxidation state of the vanadium that present at high concentrations on the equilibrium catalysts the decrease in the amount of contaminant-coke represented by Peak L was explained by the reduction of the oxidation state of vanadium. Less contaminant coke was produced after the equilibrium catalysts were pretreated using hydrogen and methane gases since reduced vanadium has lower dehydrogenation activity compared to oxidized vanadium.