Mo and W thin films and Mo/Si/Mo and W/Si/W tri-layers have been deposited by r.f. magnetron sputtering on c-Si substrates as a precursor to the fabrication of Mo/Si and W/Si multilayer X-ray mirrors. The Phase Modulated Spectroscopic Ellipsometry (SE) technique has been used for characterizing the single layer films to derive information regarding the thickness and volume fraction of voids present in the surface layers. The Mo/Si/Mo and W/Si/W tri-layer structures have been characterized by the Cross-sectional Transmission Electron Microscopy (XTEM) technique. The inter-diffusion at the interfaces of the tri-layer structures observed by the XTEM technique has been correlated to the thickness of the surface layers of the metal films obtained from the SE measurement.