A conceptual model based on the "effective voidage" around a "test particle" in a bidisperse solid-liquid fluidized bed has been proposed and has been used to interpret and predict its phase inversion behavior. The model visualizes the test particle to be surrounded by particles of "average size". An average size particle has a liquid envelope of average thickness which is different from the thickness of the liquid envelope surrounding the test particle. The effective voidage (epsilon(ff)) enjoyed by a particle of this kind has been expressed as a function of the volume fractions of individual particles in the bed. The two equations for the effective voidage (epsilon(eff)) for the two types of particles can be solved numerically for the local concentration pairs in the bed. The solution leads to the prediction of the "critical velocity" or the "inversion velocity" of the system. It also allows computation of the particle concentration profiles as well as the heights of different sections of the binary solid-liquid fluidized bed. The computed critical velocities for 19 systems studied by different workers are in excellent agreement with reported experimental data. The model is fully predictive and does not necessarily require any experimental bed expansion data for the prediction of the inversion velocity and other parameters.