We have performed a computational study on the properties of a series of heterocycles bearing two adjacent heteroatoms, focusing on the structures and electronic properties of their first excited triplet states. If the heteroatoms are both heavy chalcogens (S, Se, or Te) or isoelectronic species, then the lowest excited triplet state usually has (pi*, sigma*) character. The triplet energies are fairly low (30-50 kcal mol(-1)). The (pi*, sigma*) triplet states are characterized by a significantly lengthened bond between the two heteroatoms. Thus, in 1,2-dithiolane (1b), the S-S bond length is calculated to be 2.088 angstrom in the singlet ground state and 2.568 angstrom in the first triplet excited state. The spin density is predicted to be localized almost exclusively on the sulfur atoms. Replacing one heavy chalcogen atom by an oxygen atom or an NR group results in a significant destabilization of the (pi*, sigma*) triplet excited state, which then no longer is lower in energy than an openchain biradical. The size of the heterocyclic ring also contributes to the stability of the (pi*, sigma*) triplet state, with five-membered rings being more favorable than six-membered rings. Benzoannulation, finally, usually lowers the energy of the (pi*, sigma*) triplet excited states. If one of the heteroatoms is an oxygen or nitrogen atom, however, the corresponding lowest triplet states are better described as sigma,pi-biradicals.