Experiments with a focus on the impact of polydispersity on clustering characteristics (namely, appearance probability, duration, and frequency) of Geldart Group B particles in a circulating fluidized bed (CFB) riser have been performed. Three mixtures are considered: (i) a density-difference binary mixture, with species of different material density (rho(s)) but similar particle sizes (d(ave)), (ii) a size-difference binary mixture, with species of different d(ave) but similar material density rho(s), and (iii) a continuous particle size distribution (PSD). Local cluster information spanning the entire riser was obtained over a range of operating conditions using a fiber optic probe. Results show that cluster trends for the binary mixtures are similar to those reported in the companion work for monodisperse materials (Chew et al., this issue) on two counts. First, local riser position has a significant influence on all three cluster characteristics, while effects of operating condition and material type are secondary. Second, among the three cluster characteristics, the cluster appearance probability is most influenced by local position, and least affected by operating condition and material type. Furthermore, the density-difference binary mixture exhibits a distinctly lower cluster duration than either of its constituent components. In contrast, the size-difference binary mixture has a cluster duration profile that mimics one constituent component, while the frequency profile mimics the other. Comparing the two binary mixtures at any riser location, the density-difference binary mixture has lower cluster duration and higher frequency than the size-difference binary mixture regardless of local position. Finally, with respect to the continuous PSD, which was investigated under a wider range of operating conditions, the effect of operating condition is more apparent. This deviation may be due to an inherent behavioral difference between binary mixture and continuous PSD and/or to the wider range of operating conditions examined. (C) 2011 Elsevier Ltd. All rights reserved.