Gas-solids circulating fluidized bed (CFB) experiments have been carried out, with a focus on understanding the impact of polydispersity on cluster characteristics in a dilute riser. Two categories of polydispersity were studied: continuous particle size distributions (PSDs) of varying distribution widths, and binary mixtures of varying compositions. Video images show that particle clusters exist even in these very dilute systems (solid loading m = 0.03-0.29). Local measurements were acquired using two independent instruments - a fiber optic probe and a high-speed video camera - with the cluster trends obtained from both are in qualitative agreement with each other. Cluster characteristics extracted from the measurements include appearance probability, duration and frequency. Results show that: (i) axial position is the strongest influence on radial profiles of cluster duration and frequency, but has negligible effect on cluster appearance probability, (ii) profile shapes are invariant with height, although magnitudes of cluster duration and frequency changes with height, and (iii) effects of both the widths of continuous PSDs and the compositions of binary mixtures are observed at the riser bottom for cluster duration and at the riser top for cluster frequency, though are insignificant for the appearance probability. (C) 2011 Elsevier B.V. All rights reserved.