With the use of general transition state theory and density functional theory, six reference reactions that are thought to play an essential role in the thermal cracking process of 9-methylphenanthrene have been studied. At the uB3LYP/6-31G(d,p) level, the transition state structures could be located on the 0 K potential energy surface for the three propagation reactions which induce no net creation/annihilation of radicals, and the calculated activation energies and preexponential frequency factor generally correspond well to experimental values. The transition states for two termination reactions were determined by replacement of a phenanthrenic by a phenylic aromatic moiety; it followed that such model reactions represent well systems with larger aromatic units. Only for the initiation reaction the transition state could not be located; in this case the activation energy was approximated by the change in overall enthalpy.