An improved and updated model for modern fluidized bed catalytic crackers (FCC) is presented. It is based on a more detailed kinetic description of the kinetics in both the reactor and the regenerator using the full range of published data both on FCC performance and kinetic rates. It includes a complete description of CO to CO{sub 2} combustion kinetics including the effect of catalytic combustion promoters. The model describes FCC operation in both full and partial CO combustion as well as the transition from one to the other. The reactor kinetics are based on a ten-lump model which allows feed and catalyst characterization based on laboratory experiments. The model overcomes some of the deficiencies and inadequacies of present models and is in good agreement with observed trends in industrial units.