Raman spectra of a series of isomorphous hexafluorometallates (TlSbF 6; Ba MF 6, M = Si, Ge, Sn, and Ti, space group R 3m-D 5 3d, Z = 1 ) have been measured under high pressures up to 25 kbar with a new opposed sapphire anvil cell. Pressure calibration has been achieved by means of the R 1 fluorescence band of ruby. The frequency of the only Raman active lattice mode (libration of the octahedral anion) of each compound shows a higher pressure dependence than do the internal vibrations of the anions. The E g components of the ν 5 modes of the MF 6 octahedra which are coupled to the E g lattice modes because of their same symmetries and small frequency differences exhibit higher pressure sensitivities than do the uncoupled internal modes. These findings may be used as a further means for symmetry assignment. BaGeF 6 undergoes a reversible phase transition at 9.6 kbar; the split librational modes of the high-pressure phase show the highest pressure dependences of up to about 3 cm −1 · kbar −1 found in this investigation.