Experiments were carried out on downward-fl ow water-fl uidized binary mixture of spheres selected for possible layer inversion. With the two binaries tested (relatively high-density large particles mixed with lower-density smaller particles), only one of the fi ve possible steps of the layer inversion progression was clearly identifi ed for the higher liquid velocities, a mixed layer being observed at the top of the column close to the distributor. This layer manifested a bed contraction so large that its bulk density was lower than those of its constituent mono-components at the same liquid velocity, which determined its location within the column. The voidage of the mixed layer could be predicted with good accuracy (<0.7%) by regression of the Westman packed bed equation with one adjustable parameter.