The pyrolysis of pure o-dichlorobenzene at temperatures between 1100 and 1400 K produced a small set of tri- and tetrachlorobiphenyls whose relative yields appear to be related to steric factors associated with the presence of o-chlorine atoms. To evaluate the hypothesis that product stability governs the distribution of PCB isomers produced in this system, biphenyl and all 209 polychlorinated biphenyl (PCB) congeners were modeled using the semiempirical molecular orbital method AM1 (Austin modell). Relative enthalpies and entropies of PCB isomers were calculated, from which a small set of parameters that can be used for estimation by the method of group additivity was derived. These calculations suggest that PCBs produced by o-dichlorobenzene pyrolysis below 1400 K are thermodynamically distributed