A discrete population balance model, where the probabilities of particular bonds breaking within a given mer are assigned individual weights, is studied. The model represents an extension of previous population balance models and provides a framework for the analysis of a number of different scission mechanisms including pure random scission, end-chain scission, simultaneous random and end-chain scission and break-at-apoint scission. The main thrust of the work is aimed at interpreting observed degradation rates of PMMA. The model suggests that both random and end-chain scission must occur in order to reproduce the observed dependence of degradation rate with initial degree of polymerisation at low temperatures. However, at high temperatures it was found that molecule size dependence must also be incorporated in order to explain the observed behaviour. (c) 2005 Elsevier Ltd. All rights reserved.