Reactions of phenyl radicals with ethylene (R1), vinyl radicals with benzene (R2), and H-atoms with styrene (R3) are important prototype processes pertinent to the formation and degradation of aromatic hydrocarbons in high-temperature environments. Detailed mechanisms for these reactions are elucidated with the help of quantum chemical calculations at the G2M level of theory. Reactions R1-R3 initially produce chemically activated intermediates interconnected by isomerization pathways on the extended [C8H9] potential energy surface. All kinetically important transformations of these isomeric C8H9 radicals are explicitly characterized and utilized in the construction of multichannel kinetic models for reactions R1-R3. Accurate thermochemistry is evaluated for the key intermediates from detailed conformational and isodesmic analyses. An examination of the G2M energetic parameters for reactions R1-R3 and for briefly revisited C6H5 + C2H2 and C6H6 + H addition reactions reveals common theoretical deficiencies and suggests that the quality of theoretical predictions can be improved by small systematic corrections. Theoretical molecular and adjusted energetic parameters are used in a consistent way to calculate the total rate constants and product branching for reactions R1-R3 by weak collision master equation/RRKM analysis (addition channels) and transition state theory with Eckart tunneling corrections (abstraction channels). The available experimental kinetic data for reactions R1 and R2 is surveyed and found in good agreement with the best theoretical estimates.