U(Mo)/Al(Si) diffusion couples are studied to provide a better understanding of the interaction processes between the U(Mo) fuel and the Al(Si) matrix and, more specifically, to determine the molybdenum behaviour in the interaction product, close to the U(Mo) alloy. Micro-computed tomography and micro-X-ray diffraction measurements in transmission mode, coupled with scanning transmission electron microscopy and energy dispersive X-ray spectroscopy analysis show that molybdenum is present in the interaction layer, close to the U(Mo) interface, in an U4Mo(MoxSi1âˆ'x)Si2 ternary intermediate phase, mixed with the U3Si5 phase. The formation of this Mo-containing phase is accompanied by a Mo depletion process, in the underlying U(Mo) alloy, this depletion leading to a γ-U(Mo) phase local destabilisation. A strong correlation between the Si and the Mo behaviours, in the U(Mo)/Al(Si) system, is thus evidenced.