The impact of particle properties on segregation and mixing of bidisperse granular beds in a rotating horizontal drum have been studied by discrete element method (DEM) simulations. Bidispersities in radius, density, and mass have pronounced influences on the stationary mixing pattern, although they hardly affect the granules' flow regime. At 50% fill level, all beds mix well for a Froude number of approximate to 0.56, corresponding to a flow regime intermediate to cascading and cataracting, while segregation occurs both at lower (rolling and cascading regime) and higher (cataracting/centrifuging regime) Froude numbers. These observations are explained qualitatively by noticing that the angular drum velocity dictates the flow regime, which in turn determines the effectiveness and direction of four competing (de)mixing mechanisms: random collisions, buoyancy, percolation, and inertia. A further dozen particle properties have been varied, including the friction coefficients and elastic modulus, but these proved inconsequential to the steady-state degree of mixing. (c) 2013 American Institute of Chemical Engineers AIChE J, 60: 50-59, 2014