The reaction of niobium with diamond films leads to the formation of Nb2C at 700°C and to that of a carbon deficient NbC at 900°C. Both processes seem to be dominated by grain boundary diffusion, so that the growths do not proceed in smooth layers parallel to the substrate, but seemingly homogeneously throughout the thickness of the reacting metal (then Nb2C) film. The results of backscattering and X-ray diffraction analyses are discussed in terms of what is known about these two compounds with respect to their thermodynamic, kinetic and structural characteristics. The formation of ordered phases with carbon deficiency such as Nb4C3, or Nb6C5, and nucleation processes resulting from this are presumed to hinder the formation of near-stoichiometric NbC. © 1991.