The compounds forming on the surface of MgF₂, SrF₂, BaF₂, ScF₃,YF₃, and LaF₃ during storage in air were identified by Fourier-transform IR diffuse reflectance spectroscopy. The adlayer on the surface of MgF₂ was found to contain magnesium hydroxide; on the surface of BaF₂, YF₃, and LaF₃, basic carbonates were present. The mechanisms of the possible surface reactions are discussed, and a procedure is proposed which allows one to replace hydroxide-carbonate surface groups by fluoride groups and to enhance the stability of the fluoride surface to atmospheric attack.