The electric and mechanical relaxation spectra of the solutions of lithium perchlorate (LiClO4) in propylene carbonate (PC)-based mixed solvents are studied. The frequency-dependent dielectric susceptivity and electric conductivity from 0.05 to 20 GHz are determined with microwave reflectometry, and the shear relaxation from 15 to 95 MHz is measured with quartz-crystal shear impedance spectroscopy. An electric relaxation is observed at several hundreds of megahertz in a PC solution of LiClO4 at a concentration higher than 0.5 mol dm(-3), The amplitude and frequency of the relaxation become larger and lower, respectively, with an increase in the concentration of salt. With the addition Of Such less-polar cosolvents as dimethyl carbonate, 1,4-dioxane or benzene at fixed concentrations (0.5, 2.0, and 3.0 mol dm(-3)) of the salt, decreases in the amplitude and relaxation time are observed. Shear relaxation is also observed at the corresponding frequency, and the dependence on the concentration of the salts and cosolvents is similar. The zero-frequency conductivity and shear viscosity are discussed in terms of the relaxation.