One of the mechanisms proposed for the caries-reducing effect of fluoride is diffusion inhibition of OH–– groups through the 63 columns of the apatite structure thereby increasing the resistance of the crystal against acidic attack. The validity of this hypothesis was checked by investigation of the reactivity of solid solutions of hydroxyapatite and oxyapatite for oxydation during storage for 3 years in air. These solid solutions provide the same number of vacancies in the OH positions as occur in the apatite of tooth enamel, whereas the mobility of the OH–– ions is the critical quantity for its reactivity in analogy with fluorhydroxyapatite solid solutions. It was found that the solid solutions were not oxydized as their lattice parameters did not change during the storage period although the experimental period was far within the limits of the theoretical estimate. This invalidates the proposed mechanism for the caries-reducing effect of fluoride, as it proves the solid state diffusion is not important in apatites at normal tempe