The objectives of this work is to identify the relative importance of dimensionless numbers characterizing the convective/dispersive model and the, solid phase: mixing. A noninvasive radioactive particle tracking (RPT) technique is used to estimate the model parameters and generate tracer curves at various axial positions for two fluidized bed diameters. The dimensionless number representing the solid dispersion in the particle vortex-phase, 1/Pe(pw-pve), may be neglected for Pe(pw-pve) > 103 (Fick's term of the solid in the vortex-emulsion phase may be neglected); this situation represents, most of the experiments. The dimensionless standard-deviation of the velocity distribution of the solid rising within the bubble wake, sigma*(pwd), should not be neglected, but its variation may be neglected. The dimensionless numbers having the highest effect on solid mixing are them fraction of the solid that rises in the bubble wake (phi(pw)) and the ratio of the solid exchange between particle sub-phases over the mean velocity of the solid that are transported by the bubble wake (St). (c) 2007 Elsevier Ltd. All rights reserved.