The reaction dynamics of the dicarbon radical C2(a 3Πu/X1Σg+) in the singlet and triplet state with C4H6 isomers 2-butyne, 1-butyne and 1,2-butadiene were investigated at collision energies of about 26 kJ mol-1 using the crossed molecular beam technique and supported by ab initio and RRKM calculations. The reactions are all indirect, forming C 6H6 complexes through barrierless additions by dicarbon on the triplet and singlet surfaces. Isomerization of the C6H 6 reaction intermediate leads to product formation by hydrogen loss in a dicarbon-hydrogen atom exchange mechanism forming acyclic C 6H5 reaction products through loose exit transition states in overall exoergic reactions. © 2014 the Partner Organisations.