The purpose of this paper is to develop a theoretically sound basis for the equi-partition of kinetic energy (EKE) kernel recently developed by our group to describe the evolution of granule size distributions in fluidized bed granulation. The approach taken is to show first by distinct element modelling that the statistics of fluctuating velocity and thus frequency of collisions are well described by the kinetic theory of granular flow (KTGF)-that is, Maxwellian in nature. It is then possible to use KTGF to show that the size dependence of the aggregation process should indeed be given by the EKE kernel and that the rate constant depends only on the granular temperature, the particle density, the radial distribution function and the efficiency of collisions. It is shown how the kernel developed can be used to describe the evolution of a granule size distribution when low-density sodium carbonate particles (Light Ash) are sprayed with polyethylene glycol. (C) 2004 Elsevier B.V. All rights reserved.