Motivated by a lack of understanding in the chemical mechanisms of alkylaromatic pyrolysis, the thermochemistry of fused two-ring aromatic molecules and radicals was calculated in this work using the CBS-QB3 level of theory. The enthalpies of formation of some fused ring species differ by as much as 13 kcal/mol from previous estimates. New group values were defined to facilitate better thermochemistry estimates in the future and were found to match the CBS-QB3 calculated values with an average deviation of 0.4 kcal/mol and a standard deviation of 0.9 kcal/mol, a substantial improvement from previous estimation methods. We discuss the thermochemical characteristics of the various polycyclic and radical groups developed in this work.