Comminution, in general, and grinding, in particular, is extremely energy intensive and it is imperative that size reduction equipment is operated at optimum operating conditions. Reasonably accurate mathematical models are required to understand their dynamic characteristics to enable optimisation and control of such unit operations. This paper deals with the development of a phenomenological mathematical model for describing size reduction operation in a laboratory-size continuous ball mill. Breakage rate function parameters and a correlation relating the power drawn to the operating conditions such as the mill throughput and percent solids in the mill were determined using the data collected from the ball mill operation. Using this correlation and the estimated breakage model parameters, simulations were carried out to compare the experimental data with the simulated results. It is observed that the model is adequately accurate in predicting the product size distributions with a maximum error of about 5 in the prediction of particles finer than 104 micron in the mill product. Copyright ©2006 The Berkeley Electronic Press. All rights reserved.