The role of defects and nonstoichioinetries in molybdenite substrates, and their influence on the chemical reactivity at buried MoS 2Fe interfaces, were investigated using conversion electron Mössbauer spectroscopy and X-ray photoelectron spectroscopy. Defects in the molybdenite crystal structure intentionally introduced by Ar + bombardment, or from the inherent structure of small MoS 2 crystallites, provide pathways for the diffusion and reaction of surface iron into the bulk of MoS 2 to form FeMo 2S 4. In comparison, iron overlayers do not react appreciably with undamaged molybdenite single crystals. Excess sulfur within molybdenite, when present, migrates towards deposited iron overlayers where it forms Fe 1− x s (pyrrhotite) and FeS (troilite). Annealing temperature determines the relative fraction of pyrrhotite to troilite and the orientational relationship between the ĉ-axis of the iron sulfide and the ĉ-axis of the molybdenite substrate. The stoichiometry of the molybdenite substrate, the presence of defects, and the annealing temperature provide a number of means to adjust the properties of the MoS 2Fe interface. Therefore, it may be possible to optimize/control processing conditions so as to impact either the tribochemical or catalytic properties of this important materials.