Using an optical fiber particle concentration measuring system, an extensive study was carried out of voidage profiles in a circulating fluidized bed riser of square cross-section. Both lateral and axial voidage profiles were obtained. Particle concentration is not necessarily lowest at the axis of the riser, but may go through a minimum between the wall and the axis. The study also reveals the influence of the corners on the voidage profile. Because of the exit effect, profiles of voidage at the top of the riser are asymmetric. Bimodal and even trimodal probability distributions of particle concentration are found. The intermittency index is used to characterize the heterogeneity/homogeneity of the flow.