A mathematical treatment, based largely on the work of Kapur, has been developed to obtain breakage and selection matrices from batch grinding tests which are valid for the initial period of the process. The method is illustrated using experimental results for the grinding of hydrargillite and carbon in a laboratory scale stirred bead mill for 15 min and it is shown that this restriction still allows application of the method to continuous grinding processes in the mill. The breakage and selection functions determined by the method are shown to give a good representation of the grinding kinetics and to lead to a normalized breakage function.