The reaction C + H+3 → CH+ + H2 is frequently used in models of dense interstellar cloud chemistry with the assumption that it is fast, i.e., there are no potential energy barriers inhibiting it. Ab initio molecular orbital study of the triplet CH+3 potential energy surface (triplet because the reactant carbon atom is a ground state triplet) supports this hypothesis. The reaction product is 3Π CH+; the reaction is to exothermic even though the product is not in its electronic ground state. No path has been found on the potential energy surface for C + H+3 → CH+2 + H reaction. © 1991.