The influence of carbon additions on densification during liquid phase sintering of WC-(Fe,Ni,C) hardmetals, and of subsequent heat treatments on some mechanical properties, like hardness and fracture toughness, have been investigated. The relation between total carbon content in the mixtures and final density of the specimens has been determined, together with its effect on the structure of the resulting metallic binder ph ase. In addition, the relationship between hardness and toughness and WC grain size in these hardmetals has shown to be similar to that in the traditional WC-Co system. Nevertheless, the effect of isothermal heat treatments which bring about a modification of the microstructure of the metallic binder has proved to be beneficial for increasing toughness without an appreciable influence on hardness.