Acoustic properties and short-time (10(-11) s) structural relaxation behaviour in poly(propylene glycol), PPG-4000, complexed with different concentrations of Mg(ClO4)(2), have been examined using the Brillouin scattering technique. Below T-g the Mg2+ coordinates preferably to the OH- groups of the polymeric chain ends. Above T-g the Mg2+ coordinations to ether oxygens become increasingly more important.