Using state-of-the-art instruments and electron microscopy, the interfaces in Mo/Si multilayer mirrors fabricated by UHV electron-beam evaporation and by sputtering were examined. Peak shifts were observed in the XPS spectra at growth temperatures as low as 50 C, indicating the formation of silicide at the interface. The TEM micrographs showed that, in multilayers, the majority of the silicide formed at Mo on Si interfaces, rather than at Si on Mo interfaces. At 300 C, the multilayer structure is almost completely destroyed by the reaction. Mirrors with d(Mo) of about 40 A performed much better than those with d(Mo) of about 55 A. The Auger depth profiles showed that oxygen uptake in the sputtered samples and in samples evaporated at 200 C is limited to the top Mo layer, while samples evaporated at temperatures below 200 C had oxygen penetration through at least the first few bilayers, indicating that the silicide layers act as diffusion barriers to the oxygen.