The radical cation of trimethylenemethane (TMM.+) and a derivative, 2-isopropylidenecyclopentane-1,3-diyl, were generated by electron ionization of methylenecyclopropane (MCP) and electron-ionization-induced loss of nitrogen from 7-isopropylidene-2,3-diazabicyclo[2.2.1]hept-2-ene, respectively, in a Fourier transform ion cyclotron resonance mass spectrometer. The two radical cations were isolated and their products and reaction rates with various gaseous reagents were measured. TMM.+ displays radical-type reactivity, e.g., it slowly abstracts a hydrogen atom from benzeneselenol and a thio- or selenomethyl radical from dimethyl disulfide and dimethyl diselenide, respectively. This reactivity distinguishes it from the isomeric 1,2- and 1,3-butadiene radical cations. Surprisingly, however, the TMM.+ undergoes electron transfer reactions in addition to radical reactions. The difference between the adiabatic recombination energy of TMM.+ (9.2 +/- 0.1 eV) and the adiabatic ionization energy of MCP (9.4 +/- 0.1 eV; the earlier literature estimate is less than or equal to 9.57 eV), which were experimentally determined in this work, indicates that the above electron abstraction reactions likely involve cyclization to yield MCP land not TMM) as the final neutral product. In sharp contrast to TMM.+, the sterically hindered 2-isopropylidenecyclopentane-1,3-diyl radical cation is unreactive toward most of the reagents studied, including benzeneselenol, dimethyl disulfide, and dimethyl diselenide. The recombination energy of this TMM-type radical cation land thus, the ionization energy of the 3-isopropylidenecyclopentane-1,3-diyl biradical) was measured to be 7.8 +/- 0.1 eV.