Stirred milling is continually gaining acceptance in the mineral processing industry. The IsaMill (TM) is a high intensity stirred mill with a horizontal configuration and internal classification. The present work describes the use of Positron Emission Particle Tracking (PEPT) to trace the motion of a media bead in a simplified IsaMill (TM) rig. The rig has the same inner dimensions and disc sizes as the real M20 IsaMill (TM) but is a closed unit without any flow through, no product separator and only three discs. The PEPT system has the advantage of being able to obtain detailed charge motion measurements in opaque and aggressive environments such as those encountered in grinding processes. Glass beads (3 mm) and ceramic media (3.5 mm) were tracked over a range of volumetric fillings and rotational speeds. An analysis of the resultant trajectory fields in terms of media location (occupancy), velocity, and acceleration is presented. (C) 2010 Elsevier Ltd. All rights reserved.