The different densification kinetics of metal powders is one of the most important factors negatively affecting their possible co-sintering. This technological limit is further related to the relative fraction of the two powders, their shape and relative mean size. A decisive parameter, with this respect, is the so called particle size ratio defined as the ratio of the average particle size of the two powders. In this work the spark plasma co-sintering behaviour of an 80%vol AISI H13-20%vol AISI M3:2 blend is studied by considering nine different particle size ratios (d(H13)/d(M3:2)). The matrix of the relatively softer tool steel is strengthened by high speed steel. The highest values of density, hardness and fracture toughness are related to the smaller size of H13 particles (<45 mu m), the influence of particle size ratio being not so relevant. For bigger H13 size (125, 250 mu m), particle size ratios <1 promote a dispersion of isolated hard M3:2 particles, good densification and final high density. Increasing ratios evidence the formation of hard particles aggregates and a percolative network, respectively, which hinder densification. Above a critical value, however, the particle size ratio promotes densification in view of the lower "excluded volume" by smaller hard particles. (C) 2012 Elsevier B.V. All rights reserved.