The formation of the buried, 7 7-reconstructed Si(111)-Pb interface has been studied by X-ray diffraction. Oscillations in the reflected intensity confirm that, at 100 K, the Pb grows layer-by-layer beyond a critical coverage of 6 ml. Measurements of the specular reflectivity show that, at a coverage of 8.4 ml, the film is simply well-ordered Pb(111) with an under occupied first layer consistent with the preservation of the 7 7 reconstruction at the buried interface. At low coverages, growth oscillations and specular reflectivity data reveal significant disorder.