The description of particulate degradation using a matrix equation is a potentially useful tool in modelling degradation processes. One of the main constraints in adopting this approach is measuring the relevant breakage matrix, especially where degradation of particles in a given size class is influenced by the presence of particles in other size classes. This paper describes a technique for quantifying such intra-mixture interactions in degradation processes, hence calculating the full breakage matrix for interacting mixtures, for a relatively modest increase in necessary observational effort. The result indicates that the breakage matrix approach may be more useful and versatile than previously realised. (C) 2004 Elsevier B.V. All rights reserved.