The [H-2, C-2, O-2](+) potential energy surface was explored through the use of C2 high-level ab initio calculations. The global minimum can be viewed as the result of inserting the C+ monocation into the C=O bond of formic acid. However, the products of the C+ + HCOOH reactions do not arise from the unimolecular decomposition of this species, but from the structures that correspond to the attachment of C+ either to the carbonyl oxygen atom or to the OH group of the neutral. Both attachment processes are almost equally exothermic and yield HCO+, HCO., and COH. as dominant products. This is in agreement with the fact that HCO+ is the sole product molecular ion detected in this reaction. The unimolecular decomposition of the global minimum would yield, as the dominant product, the HCOH+ cation, which is not experimentally observed. This is consistent with our results, which show that this process cannot compete with the previous ones. The two functional groups present in formic acid perturb each other to a significant extent, and C+ + formic acid reactions present some interesting peculiarities with respect to C+ + formaldehyde and C+ + methanol reactions. Significant differences are also observed with respect to the analogous Si+ reactions.