CCSD(T)/6-311G * * //QCISD/6-311G * * calculations on the concerted [2 + 2 + 2] trimerization of ethyne to benzene yield Delta H-trim(o) (HC=CH) = - 140.2 kcal/mol and Delta H-act(o)(HC=CH) = 53.1 kcal/mol. The corresponding transition state (TS) possesses C-2 symmetry, although both the planar D-3h and nonplanar D-3 structures are negligibly higher in energy, indicating extreme flatness of the potential energy hypersurface along the distortion paths. The analogous trimerizations of HC=CC1 and CIC=CC1 are predicted to be considerably more exothermic. As the respective TSs cannot be located and the planar pseudo-TSs that possess several imaginary vibrational frequencies are associated with high reaction barriers, the concerted mechanism can be ruled our for these reactions.