Two sets of isomer products of pyrene pyrolysis at high temperatures are the six bipyrenes and their five cyclodehydrogenation products. Thermodynamic distributions of these isomer sets were computed using the AM1 semiempirical molecular model. Two properties of molecular structure, symmetry and steric hindrance associated with bonding between pyrene units, were found to govern these thermodynamic distributions. Distributions produced by pyrene pyrolysis, on the other hand, support a simple kinetic model of aryl addition and cyclodehydrogenation pathways in which reaction path degeneracy is a first-order effect and steric hindrance is of secondary importance.