Specular x‐ray reflections originating at grazing incidence at the boundary planes of thin laminated structures interfere to give the known Kiessig fringes. The change of this interference pattern during diffusion in thin multilayers is used for the study of phase formation in Au‐Al thin film couples. Layered growth of Au2Al obeying a parabolic growth relation was observed in the temperature range from 50 to 80 °C. The activation energy of the process was found to be 1.05 eV.