The layer-inversion behavior of down-flow binary solid-liquid fluidized beds is predicted using the property-averaging approach. The binary pair in this case consists of a larger solid species which is also heavier than its smaller counterpart, while both are lighter than the fluidizing medium. The model is based on using the generalized Richardson-Zaki correlation for evaluation of the bed void fraction wherein mean values of particle properties are used. However, unlike the maximum bulk density condition for the conventional up-flow binary solid fluidized bed, the model is based on a minimum bulk density condition for occurrence of layer inversion. This is due to the fact that the volume contraction phenomenon associated with the mixing of unequal solid species leads to a decrease in bulk density of the bed. Model predictions are also compared using the limited data available in the literature. Predictions are consistent with the observed mixing behavior.