A combined experimental and theoretical study of the energy loss of protons in fluorides is presented. The measurements were performed in fresh Al F3 evaporated in situ on self-supported C foils, extending the low-energy range from 25 down to 0.7 keV. The transmission method is used in combination with time-of-flight spectrometry. The experimental stopping power increases almost linearly with the mean projectile velocity, showing a velocity threshold at about 0.1 a.u. These features are well reproduced by a model based on quantum scattering theory that takes into account the velocity distribution and the excitation of the active 2p electrons in the F anions, and the properties of the electronic bands of the insulators. The model also reproduces the threshold behavior measured previously on LiF. © 2007 The American Physical Society.