Crossed molecular beam experiments were utilized to untangle the reaction dynamics to form 1-phenyl-methylacetylene [CH3CCC6H5] and 1-phenylallene [C6H5HCCCH2] in the reactions of phenyl radicals with methylacetylene and allene, respectively, over a range of collision energies from 91.4 to 161.1 kJ mol(-1). Both reactions proceed via indirect scattering dynamics and are initiated by an addition of the phenyl radical to the terminal carbon atom of the methylacetylene and allene reactants to form short-lived doublet C9H9 collision complexes CH3CCHC6H5 and C6H5H2CCCH2. Studies with isotopically labeled reactants and the information on the energetics of the reactions depict that the energy randomization in the decomposing intermediates is incomplete. The collision complexes undergo atomic hydrogen losses via tight exit transition states leading to 1-phenyl-methylacetylene [CH3CCC6H5] and 1-phenylallene [C6H5HCCCH2]. The possible role of both C9H8 isomers as precursors to PAHs in combustion flames and in the chemistry of circumstellar envelopes of dying carbon stars is discussed.