Non-aqueous solutions of lithium and sodium perchlorates have been investigated by IR and Raman spectroscopy. The ³⁵ClO₄ and ³⁷ClO₄ isotopomers induce an asymmetry or a splitting of ν₃ and ν₄ bands. All spectra are interpreted by assuming that the bands of possible solvent-separated species are at the same wavenumbers as if they were free ions (‘spectroscopically free ions’). In ionizing solvents (dimethylsulfoxide, N,N-dimethylformamide, acetonitrile or acetone) these perchlorates are partly or completely dissociated into free ions while in associating solvents (esters, cyclic ethers or nitromethane) LiClO₄ is in part dimerized. In nitromethane, LiClO₄ ion pairs are slightly dissociated into free ions at low concentrations and dimerized at high concentrations. The coordination of ClO₄ in the ion pairs is either monodentate (LiClO₄) or bidentate (NaClO₄). Both ClO₄ groups in the LiClO₄ dimer are linked by two lithium bridges into a centrosymmetrical structure.