ortho-Phenylene-bisnitrene (2) and -carbenonitrene (3) were generated in an inert matrix at low temperature, characterized by IR spectroscopy, and identified with the help of DFT calculations. Their thermal and photochemical reactivity was examined in the matrix, and it was found that both 2 and 3 ring-open to 1,4-substituted butadienes. Calculations (DFT and MCSCF) on the lowest singlet, triplet, and quintet states of 2, 3, and the all-carbon analogue ortho-phenylene-biscarbene (4) are also reported. All three species are found to have singlet ground states. The S-T splitting is small (2-3 kcal mol(-1)), but the quintet states lie siginificantly higher in energy (20-25 kcal mol(-1)). The ring-opening reactions of 2-4 (to give 1,4-substituted butadienes) as well as the ring-closure reactions (to give benzocyclobutadiene derivatives) were also investigated computationally. The calculated barriers for the ring-opening reactions are in reasonable agreement with the experimentally obtained activation energies for 2 and 3. Calculations also suggest that 4, unlike its heteroanalogues 2 and 3, has a stronger preference for the ring-closure reaction (to form benzocyclobutadiene) rather than for the ring-opening reaction.