Molybdenum was deposited onto a H-saturated surface of high quality polycrystalline HF-CVD diamond films in UHV. The interface structure and its composition were studied by XRD and RES in the temperature range 100 °C-800 °C. Interdiffusion and formation of a hexagonal Mo 2 C carbide were detected after annealing at T ≥ 550 °C. The solid state reaction leading to the bulk carbide formation is, probably, limited by carbon diffusion through the interface.