Phase-graded carbides that exhibit high hardness and corrosion resistance on the surface and high fracture toughness in the bulk are useful in severe wear applications. A method for producing hard -TaCy layer on the surface of a tough -Ta4C3-x substrate by carburization is described in this study. The -TaCy layer exhibited parabolic growth kinetics in the temperature and gas-flow regimes investigated in this study. Analyses of the rate constants using a model of mass transport through two phases in equilibrium at the interface indicated diffusion of carbon through the product layer was the rate-controlling step with diffusivities (1-5x10(-15)m(2)/s in the 1623-1773K range) and enthalpy (213-217 kJ/mol) matching those for carbon diffusion in -TaCy. Nanohardness of the transformed -TaCy layer increased as a function of distance from the carburized surface, with a maximum hardness occurring near the / interface. Measurements of lattice parameters at different depths in the -TaCy layer indicated that the hardness increase with depth was related to the increasing carbon deficiency of the -TaCy phase through the thickness.