The fluidization behavior of binary mixture differing in size in the gas bubbling fluidized bed is experimentally and theoretically studied. The segregation phenomena are analyzed, and the relevancy of the pressure drop profile of binary mixture to the definition of its minimum fluidization velocity is discussed. The distributions of mass fraction of particles along the bed height are measured, and the profiles of the mean particle diameters of binary mixture are determined. A multi-fluid gas-solid flow model is presented where equations are derived from the kinetic theory of granular flows. Separate transport equations are constructed for each of the particle class, allowing for the interaction between size classes, as well as the momentum and energy are exchanged between the respective classes and the carrier gas. The segregations of the mean diameter are predicted. The numerical results are analyzed and compared with experimental data. (C) 2003 Elsevier B.V. All rights reserved.