Structural and energetic properties are predicted for the 21 didehydroquinolinium ion isomers and 21 didehydroisoquinolinium ion isomers in their lowest-energy singlet and triplet states by using density functional and multireference second-order perturbation theories. Singlet-triplet splittings and biradical stabilization energies are examined to gain insight into the degree of interaction between the biradical centers, with comparison being made to analogous didehydronaphthalenes and didehydropyridines.