The mid-infrared spectra of the three polymorphs (clinoenstatite, ilmenite, perovskite) of MgSiO3 MgGeO3, and MnGeO3 have been recorded. The comparison between the different spectra shows that the isomorphic substitution of Si by Ge in MgSiO3 (or of Si by Ge and Mg by Mn) produces a shift of infrared bands toward lower wave numbers, the shift being larger for the substition of larger cations. Assuming that the highest frequency in each spectrum is associated with the shortest bond (Si-O or Ge-O) in the corresponding structure, we find that the frequency associated with the stretching of the bond, is given by the following empirical law: &Lgr; r7/2 μ-1/5=3810, where &ngr; is the strenghing frequency (in cm-1), μ is the reduced mass (in grams) of the oscillator Si-O (in silicate phases) or Ge-O (in germanate phases), and r is the cation/oxygen distance (in angstr¿ms) in the given polyhedra. This relation can be extended to all frequencies of all the vibrational spectra studied. Such a relation, derived from harmonic oscillator theory, allows the germanate phases (52 bands) from those of the isomorphous silicates, or vice versa. ¿ American Geophysical Union 1989