B4C diffusion barrier layers are often introduced into Mo/Si multilayered films for enhancement of thermal stability. We observe that such multilayered films exhibit both period expansion and period compaction upon annealing at temperatures below 300 °C, depending on the annealing temperature and time. Using in-situ grazing incidence x-ray reflection measurements during sequential annealing, we resolved picometer periodicity changes in Mo/B4C/Si/B4C, Si/B4C, and Mo/B4C multilayer films, and show that the two opposite period-change effects are a result of interaction of Si with B4C layers, leading to expansion, and MoxSiy formation, leading to compaction. The study of Si/B and Si/C multilayer sub-systems suggests that the cause of expansion is the formation of relatively low density SixBy compounds at the Si-B4C interface. Although the Mo-B4C interface seems to be stable based on reflectometry data, other techniques such as depth profiling x-ray photoelectron spectroscopy and wide angle x-ray diffraction measurements show that Mo and B4C actually intermix.