Abstract <p>The reflective and structural characteristics of a multilayer Ni(80)Mo(20)/C system, which is promising for the manufacture of Göbel mirrors and suppression of the Cu<i>K</i>_β emission line (λ = 0.139 nm), have been studied for the first time. The optimal ratio of Ni(80)Mo(20) and C materials has been determined to achieve the best reflectivity at a wavelength of λ = 0.154 nm (Cu<i>K</i>_α1 emission line). The reflection coefficient for periods <i>d</i> = 41.5 and 33.5 Å was <i>R</i> ≥ 69%. The positive effect of vacuum annealing of the Ni(80)Mo(20)/C structure, consisting in an increase in the first-order reflection coefficient, has been shown. The increase in reflectivity could be due to a decrease in the thickness of the transition regions and the “decompression” of the carbon layers, which is accompanied by an increase in the thickness of these layers, and an increase in the X-ray optical contrast at the interfaces. It has been found that vacuum annealing of the multilayer Ni(80)Mo(20)/C structure at temperatures up to 320°C does not significantly affect the distribution of local radiation grazing angles over the substrate area.</p>