The crystal structures of the tungsten monocarbide δ-WC and the disordered lower carbide β-W2C are studied. Using magnetic susceptibility measurements, the hexagonal carbide δ -WC is shown to be stable from 300 to 1200 K. The sequence of phase transformations associated with β-W2C ordering is analyzed. The temperature and composition stability limits of the cubic carbide γ-WC1 –x are evaluated, and the first data are presented on the variation of its lattice parameter with composition. An optimized W–C phase diagram is proposed which takes into account detailed structural and phase-equilibrium data for tungsten carbides.