Ball milling of Mo in nitrogen and ammonia gases has been studied. After heating, the formation of an intriguing ternary compound, Fe3Mo3C1−x , was observed as a result of Fe (and C) contamination from the milling apparatus. For a comparison, mechanical alloying of Fe–Mo–C powders with a stoichiometric composition of Fe3Mo3C was also performed and again an Fe–Mo–C alloy was obtained after milling.Thermal analysis shows a single transition peak for the formation of both the Fe3Mo3C phase in the latter case and also for Fe3Mo3C1−x formed as a result of contamination from the mill. The lattice parameter for the Fe3Mo3C1−x phase obtained by Mo‐gas reaction is smaller than for the Fe3Mo3C phase obtained by mechanical alloying of stoichiometric Fe–Mo–C powders. It is suggested that, in the Fe3Mo3C1−x phase produced by milling of Mo in gases, a high concentration of C vacancies exists.