Pulse radiolysis of 1,2-dichloroethane solutions of aromatic hydrocarbons was undertaken in the presence of Bu₄NPF₆ (Bu=butyl) to investigate the effect of ion pairing of the solute radical cations with PF₆^-. For most of the radical cations the dacay, which is due to neutralization with Cl^-, is retarded by the ion pairing. The rate constants, k_f and k_p, for the neutralization reactions in the free-ion and ion-paired states, respectively, were determined. The radical cations are classified into three groups according to the magnitude of the k_f/k_p ratios, which are 1.0-1.2, 2.3-3.3, and 4.6-5.6 for the three groups. The classification has been interpreted in terms of the structure and charge distribution of the radical cations. A similar pulse radiolysis study was undertaken for tetrahydrofuran (THF) solutions of the aromatic hydrocarbons in the presence of Bu₄NPF₆ and NaBPh₄ (Ph=phenyl), where the solute radical anions are generated and form ion pairs with Bu₄N⁺ and Na⁺. Spectral shifts to shorter wavelengths caused by the ion pairing are appreciable for the radical anions. The decay of the radical anions, which is due to neturalization with THF(H⁺), is also apprecialbly retarded in the presence of the salts; the k_f/k_p raios are 22-103. This is due mainly to a strong interaction of the small and charge-localized THF(H⁺) with the counter ions from the salts. In acetonitrile solution, the peak position of the absorption spectra and the decay rate of the radical anions were not affected by the addition of the salts. This indicates that the ion pairing is not important in the polar solvent. Dependence of the spectral shifts on the size of the radical anions is discussed for the radical anions of all-trans α-ω-diphenyl-substitued polyenes. The effect of various quaternary ammonium salts has also been investigated.