Upon ionization by gamma-irradiation in frozen CFCl3, or by X-irradiation in an Ar matrix, 2,2,3,3-tetramethylmethylenecyclopropane (MCP-Me-4) readily undergoes ring opening to yield the radical cation of 1, 1, 2,2-tetramethyltrimethylene methane (TMM-Me-4). The hyperfine-coupling constants for TMM-Me4(.divided by) are (mT) -1.99 (2H), +0.53 (6H), and +0.19 (6H), and the singly occupied orbital closely resembles one of the two degenerate nonbonding pi-MOs (NBMOs) of trimethylenemethane (TMM). Due to the expected effect of the methyl substituents, this "symmetric" NBMO, psi(2divided by) (b(1)), is energetically favored relative to its "antisymmetric" counterpart, psi(2) - (a(2)), so that the ground state assumes a structure with B-2(1) symmetry in the C-2v point group. Calculations show that the ring opening in the primary radical cation MCP-Me4(.+) to yield TMM-Me4(.+) is spontaneous, whereas in the parent system (MCP.+ --> TMM.+) a low barrier does exist. In contrast to the previously investigated case of the radical cation of tetramethyleneethane, the "electromer" of TMM-Me-4(.divided by), in which the unpaired electron occupies psi(2-), cannot be attained photochemically.