The stability of CaF2/Si(111) and Al/CaF2/Si(111) interfaces under irradiation by light or electrons and upon thermal annealing was investigated by photoelectron spectroscopy and scanning‐tunneling microscopy. Deposition of CaF2on Si(111)7×7 at 700 °C leads to a shift of the Fermi‐level position (EF) towards the Si valence‐band maximum (VBM), while subsequent light irradiation results in a back‐shift ofEFtowards its initial value. Upon flashing the irradiated surface at 700 °C, it shifts again towards the VBM. The variation ofEFis assigned to a competition of interface states with surface states related to irradiation‐induced fluorine vacancies. These surface defects are observed as localized surface charges in the scanning tunneling microscopy images. A thin epitaxial Al layer on top of the CaF2surface strongly reduces the irradiation‐induced effects and leads to a stabilization ofEFat 0.30 eV above the VBM.