One of the most important considerations in the design and manufacture of multilayer mirrors is material compatibility. Ideally, the materials selected should not chemically react or interdiffuse thereby avoiding the formation of second phases, enhancing the smoothness of the deposited layer interface and hence enhancing the reflectivity of the optic. It is also possible to increase the reflectivity by decreasing the thickness of the more absorbing layers and increasing the thickness of the less absorbing layers, for any given periodicity. The optimum thickness of each layer, however, is rigorously related to the specific materials as well as the x-ray wavelength being used. The transparency of the spacing (low absorption) layers can thus be augmented by materials of low absorption that are thermodynamically stable and which deposit in a smooth continuous fashion. Whereas the use of boron as opposed to carbon as a spacing material provides lower absorption, it is typically inferior in providing as low an interfacial roughness in multilayer form. For example, in rhenium-tungsten/carbon multilayers, produced by evaporation, a typical rough-ness of <0.3 nm is obtained versus 0.7 nm for rhenium-tungsten/boron multilayers. Our initial findings with B4C layers indicate deposits which show layer smoothness exceeding those containing carbon with the advantage of greater stability, lower absorption and therefore greater reflectivity. The manufacture, microstructure and reflectivity of W/B4C multilayers will be the focus of this presentation.