Zeolite catalysts are widely used in oil refinery and petrochemical industries. Fluid catalytic cracking (FCC) catalysts used in a refinery consist of Y zeolite, a silica-alumina matrix and a binder. In this study, spent FCC catalysts were prepared by cracking sour imported heavy gas oil (SIHGO) in a microactivity test unit. The total amount of coke and the hydrogen-to-carbon ratio (H/C) in the coke for spent FCC catalysts contaminated with metals were determined using temperature-programmed oxidation (TPO). Total H/C ratios of the coke on FCC catalysts were found to be in the range of 0.4 to 1, indicating the majority of the coke consists of polyaromatic species. H/C ratio decreased with increasing coke contents on the catalysts. This ratio was found to be higher for the catalyst with high metal concentration compared to the catalyst with relatively low metal concentration. The high H/C ratio for highly contaminated FCC catalyst was attributed to the formation of hydrogen rich coke by hydrogenation reactions catalyzed by the contaminant metals on the catalyst. After hydrogen pretreatment both coke amount and H/C ratio decreased significantly. This was due to the decrease in the hydrogenation activities of the contaminant-metals in their reduced forms.