Gas-phase ion chromatography experiments with variable injection energies were performed on C-7+ made by laser vaporization of graphite and by collision-induced dissociation of C-10+ and C-12+. In all cases, the C-7+ cyclic isomer dominates over the linear isomer at lowest injection energies, indicating the cyclic isomer is the most stable form. This result is consistent with ab initio calculations and a kinetic model developed here.