The mixing and segregation of particles of various types in gas–solid fluidized beds is a common phenomenon that is observed in experimental investigations. Generally, it is necessary to understand the phenomenon of mixing and segregation in gas-fluidized beds for the optimal design operation and scale-up of many industrial processes. To gain more insight into these, bed dynamics have been studied using a fully coupled Computational Fluid Dynamics/Discrete Element Method model (CFD/DEM), in which the particles are tracked individually using Newton's law of motion, and a newly developed continuum-based Multi-Fluid Model [MFM, van Sint Annaland et al. (2009a). Chem. Eng. Sci. 64, 4222–4236]. Rigorous comparisons have been made between results from laboratory experiments and the CFD/DEM and MFM. The CFD/DEM was found to reliably predict the segregation rates in low beds, provided that an appropriate gas-particle drag relation is used that accounts for the effect of polydispersity.