The possibilities of isomerization of gas-phase cyclopropane radical cations to propene or of ring opening to the trimethylene radical cation are addressed. Structural information was obtained by collisionally ionizing various C3H6 radical cations formed over a range of ionizing energies to give C3H6 2+ which fragments to give distinctive spectra of product ions. Confirmation of the conclusions drawn from these studies was found by measuring the reactivity of the C3H6 radical cations in ion-molecule reactions studied with use of Fourier transform mass spectrometry. The neutral reagents which react selectively with C3H6 radical cations are ammonia, propene-d6, and carbon disulfide. Collisionally stabilized adducts were delineated by obtaining their collisionally activated decomposition spectra with a tandem mass spectrometer. On the basis of the ammonia reaction studied here and by others, a structural change of activated cyclopropane radical cations can be clearly established. Structural isomerization to the propene radical cation can be ruled out, however, on the basis of several pieces of evidence: reactivity with propene-d6and carbon disulfide, the structure of the cyclopropane-CS2adduct radical cation, and the collisional ionization spectra. Ring opening to the trimethylene radical cation is consistent with the results of these studies, and this process is entirely analogous to the structural change of cyclopropane radical cations in condensed phases.