The thermal stability of La/B and LaN/B multilayers was investigated. The two multilayer systems were found to have comparable subångström period expansion upon annealing in the temperature range of 250°C to 400°C. For La/B multilayers, wide angle x-ray diffraction analysis revealed that the size of crystallites present did not change significantly upon thermal treatment. Using grazing incidence x-ray reflectometry, strong change in the internal structure due to interdiffusion at the interfaces of La/B multilayers was observed after annealing. This, coupled to the unchanged crystallinity, suggested the growth of amorphous lanthanum boride interlayers. At wavelength reflectance, measurements showed that as-deposited LaN/B multilayers had an enhanced optical contrast compared with La/B. During thermal loading, the rate of diffusion-induced reflectance decrease in LaN/B multilayers was slower than in La/B. The enhanced thermal stability of LaN/B was attributed to the slower growth of LaN-B interfaces compared with La-B.