The growth behavior of B(4)C 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 B(4)C at the B(4)C-on-Mo interface compared to the B(4)C-on-Si interface. X-ray photoelectron spectroscopy (XPS) depth profiling shows that for B(4)C-on-Mo the formed stoichiometry is close to expectation (4:1 ratio), while for B(4)C-on-Si it is observed that carbon diffuses from the B(4)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 nm wavelength, where B(4)C-on-Mo behaves according to model simulations, while B(4)C-on-Si does not. The as-deposited off-stoichiometric B(4)C-on-Si interface also explains why these interfaces show poor barrier properties against temperature induced interdiffusion. We show that the stoichiometry of B(4)C at the Mo-Si interfaces is connected to the structure of the layers onto which B(4)C is grown. Because of enhanced diffusion into the amorphous Si surface, we suggest that deposited boron and carbon atoms form Si(X)B(Y) and Si(X)C(Y) compounds. The low formation enthalpy of Si(X)C(Y) ensures C depletion of any B(X)C(Y) interlayer. Only after a saturated interfacial layer is formed, does further deposition of boron and carbon atoms result in actual B(4)C formation. In contrast to the off-stoichiometric B(4)C growth on top of Si, B(4)C grown on top of Mo retains the correct stoichiometry because of the higher formation enthalpies for Mo(X)B(Y) and Mo(X)C(Y) formation and the limited diffusion depth into the (poly)-crystalline Mo surface.