The objective of this study is to derive methods on a statistical basis to describe quantitatively the mixing process of a packed bed on a forward acting grate. The packed bed was represented by spherical particles, whereby a varying size accounts for the variety of particle geometries in a combustion chamber. In order to describe accurately the motion of a packed bed e.g. its particles, the discrete element method (DEM) was applied. Thus, detailed data on all the particle's paths and velocities are available. These data were used for the two proposed methods based on particles' velocities and trajectories to assess mixing and segregation of a packed bed. Both methods describe the mixing process very satisfactorily, whereby the trajectory based method is more favourable to describe segregation. (C) 2004 Elsevier Ltd. All rights reserved.