This paper presents the modelling of co-grinding kinetics in a tumbling ball mill. This modelling is first based on the analysis of size distributions measured by means of a laser diffraction granulometer. Since co-grinding involves the presence of at least two materials, with their fragments and agglomerates, the limits of this method are shown. Consequently, scanning electron microscopy is used to establish the size distributions of the different particle families. The distributions are then expressed as a function of several statistical laws. The kinetic parameters (mean size, standard deviation and fraction) are determined, and their evolution during co-grinding is discussed. (C) 2004 Elsevier B.V. All rights reserved.