The growth behavior of ${\mathrm{B}}_4\mathrm{C}$ interlayers deposited at the interfaces of $\mathrm{Mo}/\mathrm{Si}$ multilayers was investigated using x-ray photoemission spectroscopy, x-ray reflectivity, and x-ray diffraction measurements. We report an asymmetry in the formation of ${\mathrm{B}}_4\mathrm{C}$ at the ${\mathrm{B}}_4\mathrm{C}$ -on-Mo interface compared to the ${\mathrm{B}}_4\mathrm{C}$ -on-Si interface. X-ray photoelectron spectroscopy (XPS) depth profiling shows that for ${\mathrm{B}}_4\mathrm{C}$ -on-Mo the formed stoichiometry is close to expectation ( $4:1$ ratio), while for ${\mathrm{B}}_4\mathrm{C}$ -on-Si it is observed that carbon diffuses from the ${\mathrm{B}}_4\mathrm{C}$ interfaces into the multilayer, resulting in nonstochiometric growth ( $>4:1$ ). As a result, there is a discrepancy in the optical response near $13.5\mathrm{nm}$ wavelength, where ${\mathrm{B}}_4\mathrm{C}$ -on-Mo behaves according to model simulations, while ${\mathrm{B}}_4\mathrm{C}$ -on-Si does not. The as-deposited off-stoichiometric ${\mathrm{B}}_4\mathrm{C}$ -on-Si interface also explains why these interfaces show poor barrier properties against temperature induced interdiffusion. We show that the stoichiometry of ${\mathrm{B}}_4\mathrm{C}$ at the Mo-Si interfaces is connected to the structure of the layers onto which ${\mathrm{B}}_4\mathrm{C}$ is grown. Because of enhanced diffusion into the amorphous Si surface, we suggest that deposited boron and carbon atoms form ${\mathrm{Si}}_X{\mathrm{B}}_Y$ and ${\mathrm{Si}}_X{\mathrm{C}}_Y$ compounds. The low formation enthalpy of ${\mathrm{Si}}_X{\mathrm{C}}_Y$ ensures C depletion of any ${\mathrm{B}}_X{\mathrm{C}}_Y$ interlayer. Only after a saturated interfacial layer is formed, does further deposition of boron and carbon atoms result in actual ${\mathrm{B}}_4\mathrm{C}$ formation. In contrast to the off-stoichiometric ${\mathrm{B}}_4\mathrm{C}$ growth on top of Si, ${\mathrm{B}}_4\mathrm{C}$ grown on top of Mo retains the correct stoichiometry because of the higher formation enthalpies for ${\mathrm{Mo}}_X{\mathrm{B}}_Y$ and ${\mathrm{Mo}}_X{\mathrm{C}}_Y$ formation and the limited diffusion depth into the (poly)-crystalline Mo surface.