The effect of an ultrathin B ₄C barrier layer on the content of beryllides formed as a result of W and Be mixing during deposition in a short-period W/Be multilayers, and, as a consequence, on the reflectivity of the multilayers and their thermal stability is discussed in the paper. All the multilayers were prepared by means of DC magnetron sputtering. Chemical composition analysis and evaluation of thickness extensions were carried out utilizing X-ray photoelectron spectroscopy and transmission electron microscopy, respectively. The results obtained showed that the insertion of the ultrathin B ₄C barrier layer led to a decrease in the content of beryllides in multilayers due to the formation of tungsten compounds with boron/carbon. In the case of the B ₄C deposited onto the beryllium layer it was also associated with the additional formation of Be ₂C. It was suggested that the formation of the borides and carbides is more optically favorable compared to the beryllides in the multilayer without the barrier layer, which causes an increase in reflectivity. Ex situ annealing of the multilayer structures at temperatures up to 300 °C did not provoke additional mixing of the layers, but only increased a surface BeO, which led to decreasing in reflectivity of the multilayers.