The low-lying electronic states of dioxirane (1), their ring opening to dioxymethane (2), and the dissociation of 2 into CO2 and H-2 have been investigated by means of CASSCF and MRD-CI+Q quantum chemistry calculations. The ground state of 1 is a singlet with 4 pi electrons, 1(1)A(1)(4 pi), while the ground state of 2 is a 2 pi-electron singlet, 1(1)A(1)(2 pi), lying 5.8 kcal/mol higher than 1 in energy. A 0 K activation energy of 21.4 kcal/mol is predicted for the thermal ring opening of 1 into 2, which takes place via a transition structure approximately corresponding to the crossing between the lower (1)A(1)(4 pi) and (1)A(1)(2 pi) states of both molecules. Twelve excited states have been calculated for 1 with vertical excitation energies ranging from 3.07 to 13.11 eV. The energy ordering of these states changes dramatically upon relaxation of the molecular geometries. The optimum geometries of these excited states show an angle OGO in the 106.3-120.1 degrees range, so they should be considered as excited states of 2. Minimum energy points of the intersection seam between the 1(1)A(2)(3 pi)/B-1(1)(3 pi), 1(1)B(1)(3 pi)/(1)A(1)(2 pi), 1(1)A(2)(3 pi)/(1)A(1)(4 pi), and 1(1)B(1)(3 pi)/(1)A(1)(4 pi) potential energy surfaces have been located in an angle OGO range of 91.0-104.6 degrees. The photochemical ring opening of 1 into 2 may occur through vertical excitation to either the 1(1)B(1)(3 pi) or 1(1)A(2)(3 pi) states of 1 and subsequent radiationless decay to groundstate 2 via minimum energy intersection points on the potential energy surfaces of the appropriate states. The dissociation of ground-state 2 into CO2 and H-2 is predicted to be exothermic by 105.2 kcal/mol with a 0 K activation energy of 3.2 kcal/mol, while the dissociations of the first four excited states of 2 are all predicted to be endothermic.