Ab initio HF/6-31G* and BLYP/6-31G* density functional calculations of [N]phenylenes are reported for all twelve [5]phenylenes and for helical [N]phenylenes with N = 6-8. The energetics of the [N]phenylenes are put on a systematic basis by means of a simple fragment-based additivity scheme, allowing both ab initio energies and heats of formation of larger phenylenes to be estimated without resort to extensive calculation. Helical [6]phenylene racemizes through a planar transition state with a barrier of 4-5 kcal/mol. A nonplanar [5]phenylene derivative is obtained by substitution of methyl for an interior hydrogen.