An equation for correlating regeneration process variables in fluid-catalytic cracking has been developed from pilot plant data. This equation takes into account not only the chemical-reaction rate for burning coke deposited on the catalyst but also the diffusional resistance to oxygen transfer. The resistance presumably occurs between the bubbles within the fluidized bed and the void spaces in the relatively denser mass of particles. The coefficient of mass transfer was found to be inversely proportional to the 1.5 power of the average particle diameter and directly proportional to the square of the gas mass velocity. The specific reaction-verlocity constant was found to be a function to temperature and catalyst activity as well as the nature of the feed from which the coke was deposited. Comparison of pilot plant data with commercial data suggests that nonuniform gas distribution in larger beds makes some of the catalyst ineffective.