Complementary analysis techniques are applied in this work to study the interface structure of Mo/Si multilayers. The samples are characterized by grazing incident x-ray reflectivity, x-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and extreme ultraviolet reflectivity. The results indicate that the layer thickness is controlled well with small diffusion on the interface by forming MoSi 2 . Considering MoSi 2 as the interface composition, simulating the result of our four-layer model fits well with the measured reflectivity curve at 13.5 nm.