Using surface X-ray scattering and crystal truncation rod (CTR) analysis we have studied the buried atomic structure at the Pb/Si(111) interface. In agreement with previous studies, we observe that the (7 7) reconstruction is preserved at the interface, containing many features of the Si surface reconstruction and decorated with Pb atoms adsorbed at on-top sites. The (7 7) reconstruction undergoes an irreversible transition to a (1 1) structure between 150°C and 300°C. Changes in the first-order CTR indicate that the transition from the bulk Si lattice to the 'bulk-like' Pb lattice occurs over a single Pb monolayer at the interface.