This study aims to elucidate some basic features of the granular flow field in a conical frustum-shaped vertical high shear mixer granulator by using the Positron Emission Particle Tracking technique. The tested range of particle size (d(50) = 23, 43, and 60 mu m) and shape as quantified by the angle of internal friction (26 degrees and 38 degrees) have a negligible effect on the granular flow field in the mixer granulator. Density of particle imposes great influence on the flow field where reducing density from 2600 to 700 kg m(-3) increase the tangential velocity by 1.95 time. Decreasing fill level from 3.5 to 0.9 kg increases the tangential velocity by a factor of 1.19. No obvious discrepancy in the solids motion is seen between machines of I and 5 L capacity and both machines have similar mixing and dispersion with constant tip speed criterion. (c) 2007 American Institute of Chemical Engineers.