The measurement of porosity presents a significant challenge in modelling of slurry transport in rotary mills. This is due to the aggressive environment within rotary mills. In this paper, a method of measuring the porosity of mill charge, using the positron emission particle tracking (PERT) technique, is presented. In this work, multiple particles are tracked, in turn. The packing density of each size component is proportional to the residence time distribution of its representative tracer particle, based on the ergodicity of the system. The charge porosity is a linear combination of the packing densities of individual components. The porosity is modelled as a function of mill geometric and operating parameters-mill speed and filling fraction. The results show correlations between porosity distribution and operating parameters. (C) 2010 Elsevier Ltd. All rights reserved.