Abstract: The slip velocity between particle size classes in a homogeneous bidisperse gas-solid flow is quantified using particle-resolved direct numerical simulation (PR-DNS). This slip velocity is the key characteristic of size segregation in industrial devices. The ability of current gas-particle drag models to predict this slip velocity is examined by simultaneously solving the mean momentum equations for the gas phase and dispersed phases. PR-DNS of fixed particle assemblies is then used to validate and improve the bidisperse gas-particle drag model. The drag model inferred from bidisperse fixed beds is compared with the drag force measured from PR-DNS of freely evolving bidisperse suspensions. The ability of this new model to predict the slip velocity between particle size classes in a bidisperse gas-solid flow is also examined.