AbstractThe pyrolysis of benzonitrile under nitrogen at atmospheric pressure has been studied at temperatures of 823–873 K in a flow reactor. The results demonstrate that conversion to hydrogen cyanide occurs directly by a free radical mechanism. The dominant products detected are hydrogen cyanide, monocyanodiphenyl, benzene, dicyanodiphenyl and dicyanobenzene. Reaction orders and activation energies have been determined for product formation. A reaction scheme involving three competing chain reactions in the gas phase with chain carriers H., C6H 5.and.C6H4CN is proposed to explain the observed kinetics. A mechanism is advanced for the formation of significant quantities of polymer, consistent with infra‐red spectra and elemental ana