Theinsitux‐ray reflectivity measurements during linear and isothermal annealings of a W/Si multilayer (ML) were performed, and successive disappearance and reappearance of the second and third ML Bragg maxima between 400 and 500 °C were observed. Such behavior is direct evidence of a long‐range interface shift, and was found to be connected with a substantial decrease of the electronic density of the expanding originally W layers and the ML period. Surprisingly, the changes take place without smearing‐out the interfaces. The results are explained by the Si diffusion into the W layers. ©1996 American Institute of Physics.