The Segregation Approach is used as an alternative to Population Balance Modelling (PBM) to predict particle size distributions in olefin polymerisation reactors. The Segregation Approach affords simpler formulation, more explicit integration of mixing characteristics, particle growth and initial size distribution, and easier extension to unsteady-state conditions and complex kinetics. The evolution of a non-uniformly sized feed of catalyst particles in a polymer reactor system is simulated. While the two approaches yield identical predictions of the particle size distribution, in terms of computation time the Segregation model (0.09-1.7 s) far outperforms the PBM (1.8-6.2 s). This is because the sets of differential equations required by the PBM are avoided by the Segregation Approach. Based on this result, the Segregation model is described in the context of developing a model which can be used for the real-time model-based control and grade transition trajectory optimisation of olefin polymerisation processes. (C) 2011 Elsevier Ltd. All rights reserved.