Particle aggregates play an important role in the performance of circulating fluidized beds (CFBs). This paper presents a study on the aggregate properties in a high-flux and high-density riser (10 m high and 76.2 mm i.d.), based on the solids concentration measurements using a fiber optic probe. The gas velocities ranged from 5.5 to 10 m/s and solids circulation rate between 100 and 550 kg/m(2) s. Sensitivity analysis was first conducted to establish the optimum settings for determining cluster parameters. Aggregate properties (frequency, time fraction, existence time, average solids concentration and cluster vertical dimension) were established using transient solids concentration data, based on the results of the sensitivity analysis. These parameters revealed a strong dependence of the aggregate properties on operating conditions, local mean solids concentration, radial position and axial elevation.