Fluidization of a bed of particles of Geldart's group B in the voids of a packing of coarse spheres prevents the formation of bubbles and promotes, in place of it, the particulate expansion of the solid system up to porosities over 0.8. To illustrate the characteristics of the technique, the paper presents a wide series of experiments in which solids of various sizes and densities undergo fluidization and expansion in a packed bed of spheres of 4.1 mm diameter. The dependence of bed voidage on fluidization velocity and particle properties is investigated and it is shown how the classical equation of Richardson and Zaki can be reworked to account for the frictional effects associated to the presence of the coarse packing that constitutes the confining environment in which bubble-free fluidization occurs. The analysis leads to new relationships for calculating the parameters of the correlation, thus providing a quantitative interpretation of the expansion process of interest for possible applications of the confined fluidization technique. (C) 2011 Elsevier B.V. All rights reserved.