Multiphase flows in which the particulate phase consists of particles of differing size and/or density are common in industry. Flows involving such mixtures tend to segregate and this can lead to unexpected and undesired results. Traditionally, monodisperse kinetic theories with ad hoc modifications have been applied in polydisperse solids systems. In this work, a kinetic theory model developed specifically for polydisperse systems (Iddir and Arastoopour, AIChE J., 2005) is incorporated in MFIX. The impact of the polydisperse kinetic theory model on simulations predictions of gas-solid fluidized beds with a binary particulate mixture is evaluated. Specifically, predictions obtained with the new polydisperse model, the existing ad hoc model in MFIX, and with no kinetic theory model are compared against one another and against experimental data. The influence of various forms of the drag law, as applied to binary mixtures, is also explored. Two systems are considered, namely bubbling beds and risers, which are composed of binary mixtures that differ in size and/or density are considered. For the case of the bubbling bed, both low- (near minimum fluidization velocity) and high-velocity systems are examined. For the low-velocity systems, the predicted steady state axial segregation profile corresponding to the three different treatments of the solids are essentially equivalent with one exception. At high superficial gas velocities, beyond those employed in the experimental setup, differences in the steady state axial segregation profiles between the three different treatments are evident. An examination of the axial momentum equation indicates that body force, gas-solid drag, and the gas pressure drop are dominant terms in both the low- and high-velocity systems. Accordingly, the role of gas-solid drag on segregation will be explored further. The analysis also shows that kinetic theory has little role in the predictions at low-velocities, but at high-velocities the role of kinetic theory, in particular, the role of solid-solid drag due to the relative velocity between solids phases, becomes more significant. The impact of the polydisperse kinetic theory model is further assessed in the case of dilute riser flow. Previous experiments show that, in a binary mixture of powders, larger particles tend to accumulate near the walls of the riser. The polydisperse kinetic theory model captures this radial segregation behavior, whereas the ad hoc treatment does not. An analysis of the terms in the radial momentum equations shows that the terms in the solids-solids drag expression due to gradients in solids concentration and gradients in granular temperatures are the cause of this radial segregation of particles by size.