Thermal reactivity of the ortho isomers of the titled compounds has been investigated by using the full-optimized reaction space multiconfiguration self-consistent field (MCSCF) method with the STO-3G and 6-31G(d) basis sets, followed by multireference second-order Moller-Plesset perturbation calculation (MCSCF+MP2). The MCSCF+MP2 calculations lead to the prediction that all the ortho isomers have a singlet ground state but the lowest triplet state lies above the singlet ground state by less than 3 kcal/mol. Fortunately, since the spin-orbit coupling between the two states of interest is small and the singlet ground state is more likely to be produced in the photolysis, the thermal behavior of the isomers has been analyzed on the singlet energy surface. In o-phenylenebis(methylene), the E,E stereoisomer undergoes ring-closure reaction with no energy barrier to give a bicyclic compound, while the Z,Z isomer exhibits a situation of competition between the ring-opening reaction and the CH inversion followed by the ring-closure reaction. In (o-nitrenophenyl)methylene, the E isomer exhibits a competitive feature between the ring-opening and ring-closure reactions, whereas the Z isomer reacts into a linear compound with no energy barrier. In o-phenylenebis(nitrene), a spontaneous ring-opening reaction takes place definitely with the formation of a linear compound. These calculated results are in good agreement with the available experimental evidences.