Different inner vibrational modes (stretching, bending and their combination) of the OH−ion perturbed by lattice defects induced by Mg2+in NaF have been put in evidence and studied by means of optical absorption spectroscopy in the 600–7200 cm−1wavenumber range, in the 9–300K temperature range, and for Mg2+molar concentration ranging between 10−4and 10−3. Electrical and mechanical anharmonicity effects have been detected for the stretching modes and analyzed in the framework of the anharmonic oscillator model for a diatomic molecule.