In situ small-angle X-ray reflection and wide angle X-ray diffraction of synthetic, Mo¿Si based multilayer structures were used to study layer interdiffusion dynamics at temperatures between 250 and 300 °C. The in situ reflection measurements revealed information on non-constant interdiffusion characteristics during the interlayergrowth. The activation energy for interdiffusion was found to gradually increase with growing Si-on-Mointerlayer thickness, towards a saturation level of approximately 2.5 eV. Contrary, the activation energy for interdiffusion at the Mo-on-Siinterlayer was almost constant at a value around 2.6 eV. Wide angle X-ray diffraction at different stages in the annealing cycle further showed the evolution of Mo crystallites. Evolution of these crystallites was found to be strongly correlated to the change in period thickness of the multilayer structures.