Fluidization of binary beds of dissimilar solids has place along a fluidization velocity interval bounded by the "initial" and the "final fluidization velocity" of the mixture, with segregation phenomena that continuosly change the internal distribution of its components. Varying with the relative importance of size and density differences between components, the fluidization process may follow more than one mechanism, depending on whether the process of fluidization starts from bed top or bottom. It is shown how, irrespective of the fluidization pattern exhibited by the two-solid system, the limiting velocities of its fluidization interval can be calculated with good accuracy by the same relationships, derived from the analysis of the fluidization force equilibrium. The model proposed provides a unique theoretical frame for the analysis of the fluidization behavior of any two-solid system and encompasses as a particular case the behavior of simpler mixtures, whose components differ only in density or size. (C) 2012 American Institute of Chemical Engineers AIChE J, 59: 729-735, 2013