A phenomenological model of axial solids mixing in a circulating fluidized bed is formulated. The model allows for main specific features of the process: ascending motion of particles in the core zone and their descending motion in the annular zone (inner circulation of solids); substantial changes of particle concentration, sizes of core and annular zones over the bed height; net circulation of solids and the effect of the bottom bed on the process. The validity of initial postulates is confirmed by comparison of calculated and experimental curves of mixing. (C) 2003 Elsevier Ltd. All rights reserved.