Experiments in a circulating fluidized bed (CFB) riser with Geldart Group B particles have been carried out with an emphasis on cluster characterization. In this paper, the focus is on monodisperse materials; results for polydisperse materials are contained in the companion paper (Chew et al., this issue). A fiber optic probe was used for detection of solids, with the resulting solids cluster identification accomplished using wavelet decomposition. Three cluster characteristics were evaluated - appearance probability, duration, and frequency - as a function of riser position, operating condition, and type of solids (i.e., variations in particle size and/or material density). Results indicate that the riser position has the most dominant influence on cluster characteristics with the cluster appearance probability appearing to be largely a function of riser position only, and insensitive to changes in operating condition or material type. The cluster duration and frequency are influenced by the particle material and operating condition, though only from mid-height of the riser upwards. The results presented provide a comprehensive picture of factors affecting cluster trends. (C) 2011 Elsevier Ltd. All rights reserved.