The potential energy surface of the reaction C 2H 3+H 2→C 2H 4+H→C 2H 5 has been investigated using various theoretical methods including QCISD(T), CCSD(T), RCCSD(T), Gaussian-2 (G2), and the density-functional B3LYP approach. The reaction of the vinyl radical with molecular hydrogen is shown to take place through the hydrogen atom abstraction channel leading to the formation of C 2H 4+H with the activation energy of 10.4 kcal/mol at all the G2, QCISD(T)/ 6-311+G(3df,2p), and CCSD(T)/6-311+G(3df/,2p) levels. The rate constant, calculated using the variational transition state theory with tunneling correction, k=3.68·10 -20·T 2.48·exp(-3587/T) cm 3 molecule -1 s -1, is in good agreement with the experimental estimates. C 2H 5 cannot be formed directly by inserting C 2H 3 to H 2, but can only be produced by addition of H to C 2H 4, with a barrier of 4.5-4.7 kcal/mol calculated at high levels of theory. In order to match the experimental rate constant, the activation energy needs to be adjusted to 2.8 kcal/mol. Generally, the B3LYP method is found to predict well the geometries and vibrational frequencies of various species. However, it is less reliable for energy calculations than the QCISD(T) and CCSD(T) methods.