The atomic structure at the epitaxial CaF2/Si(111) interface has been determined by high-resolution electron microscopy. As-grown layers reveal only direct Ca-Si bonding at the interface, with eightfold coordinated Ca atoms. Fluorine is preferentially removed from the interface during a rapid thermal anneal leaving fivefold coordinated Ca atoms. Removal of F results in a dramatic improvement in the electrical properties of the interface. The measured interface state density is reduced from 1013 cm-2 to 1011 cm-2.