The reactions of the dicarbon cluster C-2 in the (1)Sigmag(+) singlet ground state and (3)Pi (u) triplet state with the simplest alkene ethylene, C2H4, were unraveled under single collision conditions using the crossed molecular beam method with mass spectrometric detection. Our experimental data revealed the presence of a C-2 versus H substitution pathway leading to C4H3 isomer(s) via bound C4H4 intermediates. The crossed beam results were combined with ab initio electronic structure calculations of the relevant stationary points which disclose that the formation of the n-C4H3 (X(2)A') product can take place through different reaction pathways via three-and four-membered ring structures. Based on electronic structure and RRKM calculations, the cyclobutyne molecule in its triplet electronic state should be a key intermediate in the C-2(alpha (3)Pi (u)) reaction.