Infrared spectra of the <(nu)over bar>(4) mode of the ClO4- ion, for the systems LiClO4-polyethylene oxide dimethyl ether of average molar mass 400 (PEG-400) and LiClO4-polyethylene oxide glycol of average molar mass 400 (PEG-400), at 25 degrees C, have been determined and quantitatively described by the sum of two Gaussian-Lorentzian product functions centred at similar to 623 and similar to 635 cm(-1), respectively. Bands are assigned to spectroscopically free ClO4- (at similar to 623 cm(-1)) and to bound ClO4- (at similar to 635 cm(-1)). From the relative amplitudes of the two bands, it is concluded that the majority of the ClO4- exists out of contact with Li+, probably as solvent-separated ion pairs. This conclusion is based on a large ion-pair formation constant K-p = 4.5(2) X 10(3) for LiClO4 in PEO-400, as reported in the literature and reanalyzed in this work. Electrical conductance data for LiClO4 in PEG-400 are also reported and analyzed with current theories furnishing values of K-p for this system. Existence of dimer aggregates (Li+SxClO4-)(2), with S symbolizing an oxygen moiety of a polyether chain, is inferred from ultrasonic relaxation spectra of LiClO4-PEO systems. The presence of some (LiClO4)(2) dimeric contact species is also inferred. The ultrasonic absorption spectra show two relaxation processes followed, at high frequency, by a solvent relaxation process. Matrix analysis has been applied to the two-step dimerization process 2LiClO(4) reversible arrow LiClO4...LiClO4 reversible arrow (LiClO4)(2) (which is used to interpret the electrolyte ultrasonic relaxation processes).