A cold model of a CFB having a 0.120 m-ID and 5.75 m-high riser, was operated under a relatively wide range of gas velocity and solids mass flux. Four solids (two Ballotini, a silica sand and a spent FCC), belonging to groups A and B of Geldart's classification, were selected in order to separately investigate the effect of particle size and density on axial profiles of voidage and on radial profiles of solids mass flux. Attention was focused, in particular, on the suspension density at the bottom and exit regions of the riser.