An important characteristic of fast fluidized beds (FFB) is the tendency far the solid particles to aggregate into clusters. Such clusters can strongly affect operational characteristics such as particle holdup, pressure drop, wall heat transfer, and axial mixing. This paper pre-cents results of an experimental study of cluster characteristics, as determined by capacitance-probe measurements of instantaneous local solid concentrations in a 15-cm diameter FFB. Results are presented that indicate the parametric effects of particle size (d(p)) and superficial gas velocity (V-g) on the duration time, occurrence frequency, time-fraction of existence, and solid concentration in clusters.