Modifications induced in the optical absorption spectra of CaF 2, SrF 2 and BaF 2 single crystals by annealing at 900 °C in different atmospheres have been studied. No bulk effects appear if the annealings are performed in dry oxygen or dry nitrogen atmospheres. By annealing in water containing atmospheres both oxygen and hydroxyl impurities seem to be incorporated into the bulk samples. Surface defects associated with oxygen and carbonate ions are also observed after some of the annealing treatments.