Ab initio molecular orbital theory using basis sets up to 6-311G**, with electron correlation incorporated via configuration interaction calculations with single and double substitutions, has been used to study the structures and energies of the C,H, monocation and dication. In agreement with recent experimental observations, we find evidence for stable cyclic and linear isomers of [C,H,]+'. The cyclic structure ([HCBH]", a) represents the global minimum on the [C3Hz]+' potential energy surface. The linear isomer ([H&?W+' , a) lies somewhat higher in energy, 53 kJ mol-' above a. The calculated heat of formation for [HCCCH]" (1369 kJ mol-') is in good agreement with a recent experimental value (1377 kJ mol-'). For the [C3HZl2+ dication, the lowest energy isomer corresponds to the linear [HCCCHI'+ singlet (h). Other singlet and triplet isomers are found not to be competitive in energy. The [HCCCHI2 + dication (h) is calculated to be thermodynamically stable with respect to deprotonation and with respect to C-C cleavage into CCH' + CH+. The predicted stability is consistent with the frequent observation of [C,H,I2 + in mass spectrometric experiments. Comparison of our calculated ionization energies for the process [C,H,]+' + [C,H,]2+ with the Qmin values derived from charge-stripping experiments suggests that the ionization is accompanied by a significant change in structure.