The fabrication, by electron beam evaporation, of Mo/Si and Ta/Si mutilayers designed as soft-X-ray mirrors is described. The mirrors were characterized using surface analytical methods (RBS and sputtering in combination with AES), Cu-k(alpha) reflection, and soft-X-ray optical methods, and their soft-X-ray optical properties were correlated with microstructural characteristics. A comparison of in situ C-k reflectivity curves with calculations disclosed the existence of roughnesses at the interfaces, which can not be completely described by multiplying the reflected amplitude at each interface by a Debye-Waller factor. It was found that heating of Ta/Si samples induces a considerable change (up to and above 10 percent) in the d-spacing of multilayers, while the reflected amplitude is only reduced to two thirds of its original value.