The cation of carbon dioxide, CO⁺, was definitively detected by Latter et al. along two sight lines, the molecular cloud M17SW and the planetary nebula NGC 7027, as reported in a 1993 publication. The observations were made with the 12 m telescope at Kitt Peak National Observatory. A report from a decade earlier of the detection of CO⁺ toward the Orion Molecular Cloud by Erickson et al. has been discounted: another species is likely responsible for the single line the paper assigned to CO⁺.

Since CO⁺ is thought to be formed by ionizing CO with high energy photons, it is considered to be a good tracer of photon dominated regions and has also been detected toward PDRs such as NGC 7023 and the Orion Bar. CO⁺ has also been detected toward M82, the Cigar galaxy, following a tentative detection toward Cygnus A.

When CO is ionized to form CO+, the electron that is removed comes from the C 2s orbital, which is pushed away from the C nucleus by the other electrons on the C-O bond axies as shown in the figure.