This paper reports some experimental observations on the homogeneous expansion of three FCC catalysts with increasing temperature from ambient up to 650 degreesC. These catalysts were all characterized by a wide particle size distribution and by nonperfectly spherical particles. The Richardson-Zaki equation was used to describe the homogeneous expansion of these catalysts. Experimental expansion profiles were obtained experimentally with increasing temperature, and the parameters of the Richardson-Zaki equation, n and u(t), were estimated. Experimental n and u(t), values were compared with predictions given, respectively, by the Richardson-Zaki n = n(Re-t) correlations employed for liquids and by the drag equation for the settling of an individual particle, using the Stokes law. Experimental n and ut values were also compared with data reported in the literature. For all FCC catalysts, experimental n and u(t) values were found to be much higher than the predicted ones, with the discrepancy being greater at low temperatures. The homogeneous bed was assumed to be characterized by presence of clusters, whose diameter, d(p)*, was back-calculated from the u(t)* values. The values of d(p)*, which were obtained at temperatures above 100 degreesC, were found to be of the same order of magnitude as the surface average and volume average mean diameters, indicated as d(S) and d(V), respectively. These findings underlined the importance of defining appropriate mean particle diameters to describe the particles' systems under study. Furthermore, parallel to fluidization tests, thermal analyses were carried out to investigate possible chances in the particles' physical properties as a function of temperature. Results from these tests were compared with fluidization findings in order to highlight the role of the interparticle forces on the stable behaviour of these materials.