The 24 Valence isomers of diphosphinine, P-2(C-H)(4), have been investigated at an electron-correlated ab initio level and by density functional theory. In analogy with benzene, the three planar forms exhibit full aromaticity and are the most stable isomers on the potential surface. The next isomers, by order of stabilities, are the diphosphabenzvalenes, the Dewar diphosphabenzenes, the prismanes, and the diphosphabicyclopropenyls. They are distinctly less stable than the planar isomers, and their relative energies range from 23 all the way to 93 kcal/mol above the absolute minimum. Among the planar isomers, the most stable one is the (ortho) 1,2-diphosphabenzene, in apparent contradiction with the fact that this species has not been synthesized yet. Some lines of thought to resolve this apparent dilemma are proposed, and the relative reactivities of planar isomers are discussed in terms of their different diradical character. Strain energies are calculated for the nonplanar isomers and compared to those of the isomers of C6H6 and P-6. Tentative extrapolations to tri-, tetra-, and pentaphosphinines are proposed.