The validity of a semi-empirical equation previously developed for the yield strength evaluation of particulate composites is extended to predict the ultimate tensile strength of composites that do not show large macroscopic plastic behaviour. A good correlation is obtained between the theoretical values derived from the equation proposed and the experimental results from composites filled with both surface treated and untreated particles. A dimensionless parameter related to the filler-matrix interface is obtained and used to judge how effective a surface treatment is in order to enhance the mechanical properties of several particulate filled composites.