We report on an asymmetric localized SOMO for the methyl-substituted cyclopentane radical cation. The hfs constants of hydrogens in the ring and on methyl groups were assigned experimentally with aid of specifically deuterated compounds. Hyperfine splitting constants (hfsc) for equatorial ring hydrogens are in the range 1.9-2.6 mT (on C3 and C5), whereas for the methyl (C6) proton coaxially positioned with respect to the C1-C2 bond in the ring structure, the value is higher, 4.9 mT, verifying the localization of the unpaired electron density to the C1-C2 bond. The spin density was found in agreement with an asymmetrically distorted structure calculated using the ab initio Gaussian molecular orbital method at the UHF STO-3G level. The results imply a SOMO similar to that in the cyclopentane cation radical for which the ESR spectrum of the rigid structure shows hfsc due to two identical ring hydrogens (alpha(iso) = 2.4 mT), and consequently, the theoretical reassignment of the SOMO to the a’ in C-s or a in C-2 rather than the delocalized a" in C-s which was originally proposed can be justified also on experimental grounds. The detailed spin density distributions in various methyl-substituted cyclopentane radical cations are presented, their puckered conformations being discussed in terms of positions of cis and trans methyl substituents.