The CoSi2/Si (111) interface has been studied with the X-ray standing-wave technique. The interface (a 49A thick CoSi2 layer) has been epitaxially grown on Si (111) under ultrahigh vacuum andin situ characterized with Auger spectroscopy and low-energy electron diffraction. The perpendicular lattice mismatch between epilayer and substrate has been measured with double-crystal X-ray diffraction. The X-ray standing-wave analysis gives clear indication that the Co atoms are fivefold coordinated at the interface.