Multi-reference configuration interaction MRD-CI calculations are performed to compute the vertical electronic spectrum of linear and cyclic C 8 + clusters. The electronic spectrum of the linear C 8 + isomer is dominated by the transition 2 Π u <-X 2 Π g computed at 5.45eV. The first dipole-allowed transition of linear C 8 + ( 2 Δ u <-X 2 Π g ) is calculated at 1.69eV, whereas several transitions are calculated with medium oscillator strengths between 2.30 and 5.14eV. For the cyclic C 8 + form we computed important transitions at 1.83 and 4.20eV. In addition, the first dipole-allowed transition of cyclic isomer is computed at 0.77eV (1 2 B g <-X 2 B u ). This transition should help to identify the cyclic C 8 + species since there are no dipole-allowed transitions of the linear isomer in this energy region.