Previous studies of polycyclic aromatic hydrocarbons (PAH) from a variety of combustion and pyrolysis systems have shown that certain aspects of the PAH product distribution, such as the relative abundance of certain isomers, are invariant over a range of fuels and reactor configurations. One possible explanation is that fast isomerization, facilitated by the presence of internal and external five-membered rings, may serve to normalize the product distributions, independent of the fuel or reactor configuration. To examine this possibility, we have compared experimentally measured PAH product distributions (within isomer families) to computed theoretical equilibrium distributions-first evaluating several quantum chemical methods for thermodynamic property computation: corrected AM1 semiempirical, HF/3-21G, B3LYP/STO-3G, and B3LYP/6-31G(d). Of the four methods, the corrected AM1 method is chosen for the equilibrium computations, since its root-mean-square deviation of the computed vibrational frequencies proves to be very close to that of the higher-order HF/3-21G method and since its computation is 100 times faster-a major consideration for the large molecules (3-9 rings) of this study. Using the corrected AMI method, we have computed the Gibbs free energies and equilibrium distributions from 250 to 1500 K for the following sets of PAH isomers containing internally or externally fused five-membered rings: C16H10 = fluoranthene, aceanthrylene, and acephenanthrylene; C18H10 = cyclopent[hi]acephenanthrylene, cyclopenta[cd]fluoranthene, and benzo[ghi]fluoranthene; C20H10 = dicyclopenta[cd,fg]pyrene, dicyclopenta[cd,jk]pyrene, and dicyclopenta[cd,mn]pyrene; C28H12 = dicyclopenta[bc,ef]coronene, dicyclopenta[bc,hi]coronene, and dicyclopenta[bc,kl]coronene; C20H12 = benzo[a]fluoranthene, benzo[b]fluoranthene, benzo[j]fluoranthene, and benzo[k]fluoranthene; C28H16 = benzo[a]naphtho[2,3-j]fluoranthene, benzo[a]naphtho[2,3-k]fluoranthene, and benzo[a]naphtho[2,3-l]fluoranthene; C13H10 = fluorene, benz[e]indene, benz[f]indene, and benz[g]indene. Comparing the computed equilibrium distributions to those found experimentally in catechol (o-dihydroxybenzene) and anthracene pyrolysis products, we find close agreement only for the C16H10 isomers-corroborating previous evidence of a facile route for interconversion of internally and externally fused five-membered rings in this isomer group. Because C16H10 isomers are prominent among PAH in a wide range of pyrolysis and combustion systems, the investigation and incorporation (into PAH growth models) of C16H10 isomerization kinetics are very important. None of the other PAH isomer families investigated is found to exhibit such agreement between experimental and computed results, indicating that other isomerization mechanisms, such as ethylene migration around the PAH periphery or internal rearrangement of five-membered rings in fluoranthene benzologues, are of less significance over the time scales considered.