The phase equilibria of the system W–V–C–Co are determined experimentally in the composition range corresponding to small additions of VC x in the cemented carbides WC–Co and to larger VC x contents in the hard materials WC–(V,W)C x –Co. The results are obtained from the microanalysis of the phase compositions in samples heat-treated at 1450 and 1200 °C. Two four-phase fields, { Co solution + WC+( V, W) C x+ C } and { Co solution + WC+( V, W) C x+ η }, are determined. The narrow domain between the two four-phase fields defines the composition range suitable to the formation of the hard materials WC–(V,W)C x –Co. Some interfacial features important for the microstructure of the materials are underlined.