Coke deposited on residue-pretreating catalyst gasification with steam and steam-O-2 mixtures was investigated at temperatures between 750 and 900 degrees C under atmospheric pressure. In a steam atmosphere, major product (H-2, CO, and CO2) distribution was less affected by the temperature and yielded syngas containing H-2 and CO at approximately 56 and 33 vol %, respectively. However, major product distribution depended upon the nature of coke, and when derived from a lower Conradson carbon residue (CCR) feed, a higher hydrogen concentration was obtained in the syngas. In a steam-O-2 mixture atmosphere, primary product (H-2, CO, and CO2) distribution was significantly influenced by the temperature and O-2 concentration. Both the shrinking-core and homogeneous models match the experimental data during coke-on-catalyst (COC) steam gasification, where the apparent activation energy value is about 230 kJ/mol and the apparent reaction order with respect to gasifying agent is varied with the steam partial pressure. The experimental data of COC gasification with steam-O-2 represented using the shrinking-core model are better than those using the homogeneous model, and the apparent activation energy is approximately 98 and 90 kJ/mol for the shrinking-core and homogeneous models, respectively.