The flow of spherical particles in a tumbling blender is investigated using discrete element methods (DEM). Simulations are performed on a collection of particles that are mono-disperse and bi-disperse in size. The mono-disperse system is primarily used to assess the quality of mixing as a function of fill level and time. Results reveal that radial convection is faster than axial dispersion transport. This slow dispersive process hinders mixing performance in this geometry. We also find that both axial dispersion and radial convection worsen as the blender fill level is increased. This trend is corroborated by recent laboratory experiments performed in an identical geometry. Particle velocity profiles indicate that the flow is composed of two regions: i) a high velocity layer cascading atop ii) a nearly 'solid body' rotation region. Segregating mechanisms are investigated using bi-disperse systems, which show that small particles segregate in pockets at both extremes of the axis of rotation. (c) 2006 Published by Elsevier B.V.