The chemical reaction dynamics to form phenylmethylacetylene, C6H5CCCH3(X (1)A(＇)), via reactive collisions of the phenyl radical C6H5(X (2)A(1)) with methylacetylene, CH3CCH(X (1)A(1)), are unraveled under single collision conditions in a crossed molecular beam experiment at a collision energy of 140 kJ mol(-1). The laboratory angular distribution and time-of-flight spectra of C9H8+ at m/e=116 indicate the existence of a phenyl radical versus hydrogen replacement pathway. Partially deuterated methylacetylene, CH3CCD(X (1)A(1)), was used to identify the site of the carbon-hydrogen bond cleavage. Only the loss of the acetylenic hydrogen atom was observed; the methyl group is conserved in the reaction. Electronic structure calculations reveal that the reaction has an entrance barrier of about 17 kJ mol(-1). Forward-convolution fitting of our data shows that the chemical reaction dynamics are on the boundary between an osculating complex and a direct reaction and are governed by an initial attack of the C6H5 radical to the pi electron density of the C1 carbon atom of the methylacetylene molecule to form a short lived, highly rovibrationally excited (C6H5)HCCCH3 intermediate. The latter loses a hydrogen atom to form the phenylmethylacetylene molecule on the (2)A(＇) surface. The phenylallene isomer channel was not observed experimentally. The dynamics of the title reaction and the identification of the phenyl versus hydrogen exchange have a profound impact on combustion chemistry and chemical processes in outflows of carbon stars. For the first time, the reaction of phenyl radicals with acetylene and/or substituted acetylene is inferred experimentally as a feasible, possibly elementary reaction in the stepwise growth of polycyclic aromatic hydrocarbon precursor molecules and alkyl substituted species in high temperature environments such as photospheres of carbon stars and oxygen poor combustion systems. (C) 2000 American Institute of Physics. [S0021-9606(00)01409-4].