The component additive rule for the prediction of the volume of fluidized mixtures of binary-solids does not hold owing to the phenomenon of the volume-change of mixing. A significant contraction in the volume of the mixed bed is often observed. This phenomenon has an important bearing on the hydrodynamics of the binary-solid fluidization. In the present, the volume-change of mixing is experimentally investigated for several liquid-fluidized binary-solid mixtures for a wide range of compositions at the incipient fluidization. Seven different binaries are composed from eight different solid species that differ in their sizes as well as densities, and therefore exhibit a wide range of stratification pattern. The use of packing models reported in the literature results in rather poor predictions of the observed volume-change of mixing. Incorporating the hydrodynamic aspects of the fluidization however helps to substantially improve predictions. A two-level integration of the hydrodynamic information is implemented here. The level one involves using the hydrodynamic drag diameter evaluated from the expansion of individual solid species instead of the commonly used volume-equivalent or the packing-equivalent diameter. The level two in addition involves accounting for the expansion of either of the solid species should it occur at the incipient fluidization of the mixture. (C) 2011 Elsevier B.V. All rights reserved.