A model is presented for the prediction of the fluid dynamic behaviour of binary suspensions of solid particles fluidized by Newtonian fluids. The equations of motion for the fluid and solid phases are derived by extending the averaged two-fluid equations of change for identical spheres in Newtonian fluids developed by Anderson and Jackson and Jackson. A new closure relationship for the fluid - particle interaction force is employed and a new numerical algorithm is developed to control the solid compaction in each particle phase. The article also presents a comparison between three different equations of closure for the particle - particle drag implemented within the model. Predictions of the fluidization behavior obtained by the proposed model are validated against experimental results in terms of solid mixing and segregation, bed expansion and bubble dynamics. Two-dimensional CFD simulations are performed in a bed of rectangular geometry using ballotini with particle sizes of 200 and 350 mu m. (c) 2007 American Institute of Chemical Engineers.