This work explores the structures and energetics of linear and cyclic [N]phenylenes by ab initio calculation at the HF/6-31G* and BLYP/6-31G* density functional levels. Heats of formation were obtained from these energies by a group-equivalents method. In the acyclic case with N = 3-5, angular phenylenes are more stable than linear phenylenes. For the cyclic [6]phenylene (antikekulene), which is found to be planar, no appreciable destabilization is found relative to a linear [6]phenylene reference structure. The cyclic [4]- and [5]phenylenes adopt tub conformations. There is a simple relationship between the Delta H-f degrees of an [N]phenylene and that of the corresponding condensed benzenoid. It provides, for example, an estimate of Delta H-f degrees of C-120, archimedene, from that of C-60.