An ab initio study at the Hartree-Fock level has been carried out to determine the structure of linear and cyclic SiC 2 +. Fourth-order Møller-Plesset perturbation theory reveals that the cyclic isomer (C 2v symmetry) is the most stable, with a 2A 1 ground state. Vibrational analysis at the Hartree-Fock level confirms that, unlike cyclic SiC 2, SiC 2 + (C 2v) is a true minimum on the HF potential hypersurface.