A number of theoretical models have appeared in the literature which predict the inversion phenomenon of liquid fluidization whereby smaller denser particles in a binary mixture report to the bottom at low superficial velocities, but to the top at higher velocities. This paper reviews and categorizes these models into five groups based on the mechanisms and approaches followed. Revisions are proposed in some models for greater accuracy or consistency. The predictions of inversion velocity, i.e. the velocity at which the bed is homogeneously mixed, are then compared to available experimental data. Unfortunately, the exact temperature was not specified in most of the experimental studies, and since the predictions are sensitive to the liquid viscosity, which depends strongly on temperature, this introduces significant uncertainty into the comparison. Agreement between predictions and experimental data is within +/- 14% on average for the best models. (c) 2006 Elsevier Ltd. All rights reserved.