Multilayer Ag/Y mirrors intended for the spectral range of 9–11 nm have been investigated. The parameters of mirrors, specifically, the thicknesses of transition layers, have been determined. The efficiency of B_4C and Si barrier layers has been demonstrated. It has been shown that the properties of structures like Ag/Y and Ag/Y with B_4C and Si barriers layers have poor temporal stability. The highest reflection coefficient of 18% at a wavelength of 9.34 nm, has been observed for the Ag/Si/Y structure. The same structure has the best temporal stability.