A methodology for characterising shear rate profiles in tumbling mills based on in situ trajectory fields from Positron Emission Particle Tracking (PEPT) experiments is presented. Experiments were carried Out in a laboratory scale mill fitted with lifter bars, pulp lifters and a discharge grate, and run with radioactively labelled particles within re-circulating slurry environments. Using time-averaged velocity distributions of small PEPT tracers, a model for tangential velocity profiles along radial scan-lines is proposed from which the shear rates are derived. (C) 2010 Elsevier Ltd. All rights reserved.