We studied the structure and optical properties of B4C∕Mo∕Y∕Si multilayer systems. Using extended x-ray absorption fine structure measurements at the Y and Mo K-edge, the structure of the subnanometer thick Y layer and the underlying Mo layer were analyzed. It was found that even a 0.2 nm thick Y layer significantly reduced silicon diffusion toward Mo, thus reducing Mo silicide formation. Hard x-ray reflectometry showed that the difference in average interface roughness of the B4C∕Mo∕Y∕Si multilayer structure compared to Mo/Si and B4C∕Mo∕B4C∕Si multilayer structures was negligible. Soft x-ray reflectometry showed optical improvement of B4C∕Mo∕Y∕Si with respect to Mo/Si and B4C∕Mo∕B4C∕Si multilayer structures.