Radical anions of several alkyl-substituted buta-1,3-dienes, 4-9, and of 1,4-di-tert-butylcyclohexa-1,3-diene (11) were characterized by their hyperfine data with the use of ESR and ENDOR spectroscopy. Some of these radical anions, which are protected by bulky alkyl substitutents, could be generated by reaction of the corresponding neutral compounds with a potassium, rubidium, or cesium mirror in 1,2-dimethoxyethane (DME) and, mostly, tetrahydrofuran (THF), In particular, the radical anions of 1,4- and 2,3-di-tert-butylbuta-1,3-dienes (6 and 7, respectively) prepared by this method proved to be fairly persistent, and their ESR and ENDOR spectra were thoroughly studied in a wide temperature range. Characteristic of the hyperfine patterns of 6(.-) and 7(.-) are very large coupling constants of the alkali-metal nuclei in the counterion : a(K-39) = 0.12-0.15, a(Rb-85) = 0.40-0.84, a(Rb-87) = 1.4-2.8, and a(Cs-133) = 0.70-2.6 mT. Values of this size, unusual for counterions of hydrocarbon radical anions, point to a tight or contact ion pairing of 6(.-) and 7(.-) with the alkali-metal cations M+ (M = K, Rb, Cs). Whereas for 6(.-)/M+ in DME and THF only such tight ion pairs were observed, their coexistence with loose or solvent-separated ion pairs was noticed for 7(.-)/K+ in DME at very low temperatures. Apart from the large hyperfine splittings due to the alkali-metal nuclei, a striking feature of the tight ion pairs 7(.-)/M+ is the coupling constant of two protons in the 1,4-positions; its absolute value (ca. 0.4 mT) is much smaller than that (ca. 0.7 mT) of the corresponding protons in the radical anions of buta-1,3-dienes. Assignment of this reduced value to the endo-protons is compatible with the ESR and ENDOR spectra of 7-d(2)(.-)/M+ dideuterated in the 1-position. The structures of 6(.-)/M+ and 7(.-)/M+ were discussed with the aid of theoretical calculations. Undoubtfully, the planar trans-conformation with the bulky 1,4-tert-butyl substituents in the sterically unhindered exo-positions is generally favored for 6(.-)M(+). The counterion M+ should be situated on the 2-fold axis above or below the molecular plane of 6(.-). In 7(.-), the steric overcrowding by the 2,3-tert-butyl substituents is relieved by a strong twist about the C(2)-C(3) bond; loose and tight ion pairs 7(.-)M(+) may have different conformations. For the tight ion pairs 7(.-)/M+, a conformation should be preferred in which the counterion M+ is situated on the 2-fold axis with the two tert-butyl substituents "on the other side" of the molecule. The close contact of the alkali-metal cation M+ with either 6(.-) or 7(.-) must be promoted by the spatial arrangement of these substituents.