1. In alloys with 18% W and 25% Co the addition of carbon in amounts up to 0.74% has no effect on the temperature range of the α→γ transformation. For all alloys investigated, this range is 920–960°, since the alloys do not contain carbide phases easily dissolved during heating (M23C6, M3C). 2. No metastable M23C6 is observed in Fe-Co-W-C alloys; along with M6C, one observes M6C' with a smaller lattice constant (11.10 A for M6C and 10.89 A for M6C'). WC is observed in alloys with 0.74% C (W/C=1.57), which are observed in the Fe-W-C system only after prolonged annealing. 3. Carbon increases the secondary hardness of these alloys, although with over 0.23% C (W/C">5.1) the resistance to tempering decreases substantially, and therefore the amount of carbon should not exceed 0.23% in order to obtain good cutting properties.