The crossed molecular beam reactions of the phenyl and D5-phenyl radical with diacetylene (C(4)H(2)) was studied under single collision conditions at a collision energy of 46 kJ mol(-1). The chemical dynamics were found to be indirect and initiated by an addition of the phenyl/D5-phenyl radical with its radical center to the C1-carbon atom of the diacetylene reactant. This process involved an entrance barrier of 4 kJ mol(-1) and lead to a long lived, bound doublet radical intermediate. The latter emitted a hydrogen atom directly or after a few isomerization steps via tight exit transition states placed 20-21 kJ mol(-1) above the separated phenyldiacetylene (C(6)H(5)CCCCH) plus atomic hydrogen products. The overall reaction was determined to be exoergic by about 49 ± 26 kJ mol(-1) and 44 ± 10 kJ mol(-1) as determined experimentally and computationally, thus representing a feasible pathway to the formation of the phenyldiacetylene molecule in combustion flames of hydrocarbon fuel.