A model equation was developed to predict the superficial liquid velocity at the layer inversion point in a gas-liquid-solid three-phase fluidized bed with binary solids. The equation is based on the gas-perturbed liquid model which assumes that the solid particles in three-phase fluidized beds are fully supported by the liquid, the presence of gas bubbles in a bed reducing the available cross-sectional area and thereby increasing the interstitial liquid velocity. The superficial liquid velocity at layer inversion for the binary solids system selected was determined at superficial gas velocities up to 21.5 mm/s using a mixing index. The resulting layer inversion velocity decreased with increasing superficial gas velocity. The experimental data are consistent with our model. Predicted layer inversion velocities are compared with experimental data obtained in this study. (C) 2013 Elsevier Ltd. All rights reserved.