Abstract We report the first Brillouin scattering study of salt complexed polymers. The technique has been used to investigate the short-time (≈ 10 −11 s) local mobility of the polymer chain of salt complexed poly (propylene glycol) (PPG) and compare with that of pure PPG. A series of complexes PPG/MSCN of concentration O:M = 12:1 (where the oxygen atom O represents the number of monomers) for M = Li, Na and K were studied. A strong relaxation was demonstrated in a huge velocity dispersion and a large absorption in the temperature range 300–450 K. The phenomenon reflects the intrachain processes which, when frozen in, determine the glass transition temperature. The hypersonic velocity dispersion and the sound absorption curves are both shifted considerably to higher temperatures for the doped PPG as compared to the corresponding curves of the pure PPG. However, the results for the different salt complexes overlapped and no effect of the cation substitution could be observed for an unchanged salt concentration. Using the temperatures of the observed absorption maxima combined with the ideal glass transition temperature T 0 of the pure PPG (170 K), T 0 for the complexes was calculated to be 198 K.