Both multilayer period thickness expansion and compaction were observed in Mo/B4C multilayers upon annealing, and the physical causes for this were explored in detail. Using in situ timedependent grazing incidence X-ray reflectometry, period changes down to picometer-scale were resolved. It was shown that the changes depend on the thickness of the B4C layers, annealing temperature, and annealing time. Although strong stress relaxation during annealing was observed, it was excluded as a cause for period expansion. Auger electron spectroscopy and wide angle X-ray diffraction measurements revealed the growth of interlayers, with associated period changes influenced by the supply of B and C atoms to the growing compound interlayers. For multilayers with a Mo thickness of 3 nm, two regimes were recognized, depending on the deposited B4C thickness: in multilayers with B4C 1.5 nm, the supply of additional Mo into the already formed MoBxCy interlayer was dominant and led to densification, resulting in period compaction. For multilayers with B4C 2 nm, the B and C enrichment of interlayers formed low density compounds and yielded period expansion.