The relevance of establishing the structure of electrolyte solutions of low dielectric permittivity is briefly recalled. Microwave complex dielectric permittivities ε* = ε′ − Jε″ in the frequency range ∼1 to 130 GHz for LiClO₄ dissolved in poly(ethylene oxide)dimethyl ether of average molar mass 500 (PEO-500), in the concentration range 0.40 to 1 mol dm⁻³, at 25 °C, are reported. The data are interpreted by a Cole–Davidson distribution function. Molar electrical conductance for the same system, in the concentration range 7 × 10⁻⁴ to ∼1 mol dm⁻³, shows a minimum and a maximum as a function of the electrolyte concentration. The data, in the diluted range (⩽0.017 mol/dm³) are interpreted by the Fuoss–Onsager equation, in terms of free ions and ion-pair-species. Ultrasonic relaxation spectra in the frequency range 1 to 300 MHz and concentration range 0.1 to ∼0.4 mol dm⁻³, are interpreted by the ion-pair dimerization equilibrium 2LiClO₄ ↔ (LiClO₄)₂, the dimer being an apolar antiparallel species , thus rationalizing the depolarization of the solution with the consequent appearance of a maximum in the dielectric permittivity of the solution at high electrolyte concentration. A parallel with previous work on LiPF₆ dissolved in the same solvent is drawn.