The fluid-bed stability of three fresh FCC catalysts has been investigated both experimentally and theoretically as a function of increasing temperature. Values of the voidage at minimum bubbling conditions were obtained from 20 degreesC up to 650 degreesC, and compared with predictions given by the Foscolo and Gibilaro particle bed model. The formulation of the drag force proposed by Foscolo and Gibilaro has been generalized in order to correctly describe the homogeneous expansion of the three FCC catalysts. Consequently, the procedure followed to obtain the Foscolo and Gibilaro stability criterion has been re-examined, and a generalized expression of their criterion is proposed. Predictions obtained from the generalized particle-bed model were found in agreement with experimental data. This paper discusses also the effect of the drag and buoyancy force in predicting correct trends for epsilon (mb). To this end, results obtained using the Jean and Fan model are reported. The physical origin of the stability of these catalysts is also discussed, and the role of the hydrodynamic forces and interparticle forces is examined.