A study was made of the mixing of particulate solids fluidized in the voids of a packed bed of larger nonfluidized bodies. In such a system heat transfer rates are dependent on the movement of fluidized particles. Recently application has been made of inert fluidized particles in chemical reactor systems to improve the removal of the heat generated by the exothermic reaction of the larger particles. A measure of the particle movement was achieved in this study by the determination of diffusion coefficients for the rates of solids mixing in the lateral direction when metal shot is fluidized in the voids of spherical and cylindrical packing. Experimental data showed that the particle mixing was related to the void structure of the packed bed by a random walk model. A general correlation was made for fluidized-solids diffusivity in spherically packed beds.