The reaction between CH2 ((XB1)-B-3) + O-2 in the gas phase was investigated by carrying out CASSCF and CASPT2 calculations with the 6-31G(d,p) and 6-311+G(3df,2p) basis sets. The reaction proceeds by the addition of 02 to methylene and may follow a singlet and a triplet potential energy surface. In both cases, its fate is the carbonyl oxide (H2COO). The reaction in its singlet multiplicity is computed to be exothermic by 50.9 kcal/mol with an activation enthalpy of 1.9 kcal/mol at 298 K. The reaction in its triplet multiplicity is computed to be exothermic by 26.9 kcal/mol with an activation enthalpy of 5.4 kcal/mol at 298 K. According to classical transition state theory, the branching ratio for the reaction in its triplet state multiplicity changes from negligible at 298 K to about 26% at 1800 K, and this ratio equals the formation of atomic oxygen (0, P-3). The following unimolecular decomposition of carbonyl oxides in its singlet and triplet multiplicity was also considered.