The Raman and infrared spectra of the potassium and caesium salts of [TcX6]-- and [ReX6]-- (X = F, Cl, Br, I) have been measured in the range from 1000 to 45 cm-1. The force constants are determined in the valence force field and in the standard Urey-Bradley field on the basis of the Oh-symmetry. The fundamental vibration ν6 is calculated from the force constants. Apart from the fluoro-compounds the bond-stretching force constants of the technetium complexes are shown to be smaller than those of the analogous rhenium complexes. Except for the iodo-compounds a distortion of the octahedral structure progressive in degree towards X = F can be concluded from the splitting of degeneracies.