In this work, the hydrodynamic behavior of expanded beds was studied experimentally and theoretically. The transient response and steady-state behavior of the bed height at different flow rates were measured. The applicability of the Richardson-Zaki equation to predict the steady-state behavior is demonstrated, and a correlation is proposed to determine the dependence of the transient response of the bed height on particle and fluid properties. We have also determined the axial size distribution in the expanded state under steady conditions for four different kinds of particles. We have extended the investigation to analyze the behavior of the bed experimentally for a binary mixture of different particles. In the experiments with binary mixtures, a distinct segregation of these particles could be achieved. Size distribution analysis indicates a clear axial stratification of the two chosen particles, leading to a maximum in average particle diameter as we move along the bed in the upward direction. The data obtained in this study showed that it is possible to estimate the steady-state bed height for a single particle type as well as for mixtures of two particles. The number of empirical parameters needed to characterize the system is only two. The results of this study can be used to design expanded-bed setups using two differently functionalized particle types. © 2007 American Chemical Society.