The growth behavior of B4C interlayers deposited at the interfaces of Mo/Si multilayers was investigated using x-ray Photoemission spectroscopy, x-ray reflectivity, and x-ray diffraction measurements. We report an asymmetry in the formation of B4C at the B4C-on-Mo interface compared to the B4C-on-Si interface. X-ray Photoelectron spectroscopy (XPS) depth profiling shows that for B4C-on-Mo the formed stoichiometry is close to expectation (4:1 ratio), while for B 4C-on-Si it is observed that carbon diffuses from the B4C interfaces into the multilayer, resulting in nonstochiometric growth (>4:1). As a result, there is a discrepancy in the optical response near 13.5 nm wavelength, where B4C-on-Mo behaves according to model simulations, while B4C-on-Sí does not. The as-deposited off-stoichiometric B4C-on-Si interface also explains why these interfaces show poor barrier properties against temperature induced interdiffusion. We show that the stoichiometry of B4C at the Mo-Si interfaces is connected to the structure of the layers onto which B4C is grown. Because of enhanced diffusion into the amorphous Si surface, we suggest that deposited boron and carbon atoms form SiXBY and SiXCy compounds. The low formation enthalpy of SiXCy ensures C depletion of any B XCY interlayer. Only after a saturated interfacial layer is formed, does further deposition of boron and carbon atoms result in actual B4C formation. In contrast to the off-stoichiometric B4C growth on top of Si, B4C grown on top of Mo retains the correct stoichiometry because of the higher formation enthalpies for Mo XBY and MoXCY formation and the limited diffusion depth into the (poly)-crystalline Mo surface.