A phenomenological model of solids mixing in a circulating fluidized bed is formulated. The characteristic feature of this model is taking into account the convective flows of particles in the radial direction, which provide the observed in practice essential decrease of the concentration of particles over the riser height. It is established by comparison of calculated and experimental curves of mixing that the value of the coefficient of radial dispersion of particles lies within the range 0.0006-0.006 m(2)/s. (c) 2006 Elsevier Ltd. All rights reserved.