Water fluidization in a 210 mm diameter semi-cylindrical acrylic column of a binary solids mixture of 3.2 mm polymer beads (rho(s) = 1280 kg/m(3)) and 0.385 mm glass beads (rho(s) = 2500 kg/m(3)) at superficial liquid velocities from 18.1 to 43.1 mm/s is shown to generate layer inversion at a superficial liquid velocity, U-L, of 33.1 mm/s. Introduction of air with a superficial velocity, U-g, of 1.92 mm/s yielded a layer inversion velocity at U-L = 30.4 mm/s. The latter is explainable if it is assumed that the determinant of layer inversion is the interstitial liquid velocity and that therefore the main function of the gas in this respect is to occupy space. Mixing of the binary solids, as quantified by a mixing index applied to measured particle compositions at different levels of the fluidized bed, is shown to be greatest at the layer inversion velocity for liquid fluidization and, in general, to increase as co-current gas flow increases at a fixed value of UL. (C) 2011 Elsevier Ltd. All rights reserved.