High resolution electron microscopy has been used to determine the atomic structure at the epitaxial CaF2 /Si(111) interface before and after a rapid thermal anneal. Direct Ca-Si bonding at the interface, with 8-fold coordinated Ca atoms is observed in as-grown layers. Fluorine is preferentially removed from the interface during a rapid thermal anneal leaving 5-fold coordinated Ca atoms. A dramatic improvement in the electrical properties of the interface is observed after annealing. The measured interface state density is reduced from ≳1013 cm−2 to ≲1011cm−2. This has been correlated with the removal of F from the interface. No evidence for direct F-Si bonding is observed.