Successful separation of a precoked, equilibrium USY cracking catalyst into fractions of increasing age reveals a deactivation profile that is largely controlled by zeolite dealumination. Precoking is found to aid the density/age separation only for the youngest fractions. The observed change in Ni/V ratio with increasing "Ni age" is indicative of vanadium migration.Catalytic activity, assessed by cumene cracking on separated fractions and also by analysis of residual coke on catalyst fractions, shows a sharp decline with increasing density (age). This rapid loss of initial activity coincides with zeolite dealumination which is largely completed as a slow rate of zeolite destruction is established. Subsequent loss of crystallinity has little additional effect on activity. The associated loss of microporosity leads to an apparent increase in skeletal density with increasing age.Lattice imaging studies by TEM on a "young" fraction showed extensive regions of crystallinity with minimal evidence of crystallite fracturing. By contrast, similar investigations of an "old" fraction, combined with in situ compositional analysis, revealed small "islands" of crystallinity within a "sea" of disordered material that was once crystalline. Fracture lines at crystallite boundaries are absent. Instead, the small USY crystallites within the "old" fraction are in intimate mixture with the collapsed zeolite. Laboratory evaluation of fluidized-bed catalytic cracking (FCC) catalysts is critically dependent upon the method chosen to simulate catalyst deactivation in the commercial FCU. Conventional laboratory deactivations (high-temperature steam 0097-6156/91/0452-0109$09.75/0