It is shown that recent real-time measurements of the initial rates of decomposition of vinylacetylene behind incident shock waves together with earlier kinetic data for acetylene dimerization closely satisfy detailed balance, i.e., k_1/2k_(-1) = K_1 (Hidaka, Y.; Tanaka, K.; Suga, M. Chem. Phys. Lett. 1986, 130, 195; Skinner, G. B.; Sokoloski, E. M. J. Phys. Chem. 1960,64, 1952). The short times involved (t < 20 µs) allow molecular species, or their isomers, to take part in reactions 1 and -1 but specifically exclude free monoradical chain mechanisms. Since a one-step concerted dimerization is otherwise untenable on kinetic grounds, the intermediacy of vinylidene (H_2C=C:) in reaction 1 is proposed on the basis of ab initio calculations
