[en] A study is reported of the siliciding of pieces of superficially carburized molybdenum, and of the diffusion in a coating resulting from such a treatment. The results obtained depend essentially on the initial thickness of the carbide layer. To behave as a semi-infinite medium this layer must be very thick so that the results may be considered in terms of the classical scheme of ternary diffusion. If the carbide layer is too thin its two components will react separately in interfacial reactions with silicon. Only molybdenum reacts during siliciding to give MoSi₂ and then Mo₅Si₃ is found during annealing; at the same time carbon diffuses into the metallic substrate and reacts with it to give Mo₂C. This complication has led us to examine further the extent of diffusion in different couples in the Mo-C-Si system. (Auth.)