Mean amplitudes of vibration of a series of tetrahedral XY₄ molecules and ions (hydrides, halides, oxides and oxoanions) have been calculated using the “Method of the Characteristic Vibrations” of A. Muller. The results indicate that this method leads to very good values for most of the investigated species, and especially in the cases of high MX/MY mass ratio.Facultad de Ciencias Exactas