This paper focuses on the determination of the selection function parameters alpha, a, mu and Lambda together with the exponent factors eta and xi describing the effect of ball size on milling rate for a South African coal. A series of batch grinding tests were carried out using three media single sizes, i.e. 30.6, 38.8, and 49.2 mm. Then two ball mixtures were successively considered. The original manufacturer's recommended ball mixture was used to investigate the effect of ball size distribution on the selection function whereas the equilibrium ball mixture was used to validate the model. Results show that with the six parameters abovementioned, the charge mixture is fully characterized with about 5% deviation. Interestingly, the estimated parameters can be used in the simulator model allowing one to find the optimal ball charge distribution for a set of operational constraints. (C) 2009 Elsevier Ltd. All rights reserved.