The minimum energy path for the reaction O(lP g) + C 2~( 1 Ag) has been calculated by optimizing all relevant geometrical parameters along the approach of oxygen to ethene. A barrier of 4.7 kcallmol in the 3 A"( ... 9a' 2 -10a'3a") potential energy surface and an energy difference of 14.4 kcal/mol between the product and the fragments is found at the multireference-configuration interaction Ievel. The corresponding values at the lower-level treatment CASSCF are 9 kcaVmol for the barrier and 9 kcaL'mol for the depth of the potential; this shows the importance of inclusion of electron correlation. The barrier for CH2 rotation for the lowestenergy structure (asymmetric OC 2~) is around 5 kcaL'mol. The energy gap to the first excited state 3 A'-( ... 9a'l0a'3a'12) is found tobe 3.6 kcaL'mol in MRD-CI calculations at the ground-state minimum. Comparison with 3 CH 2 + C 2~ shows that in this system the lowest-energy surface is 3 A', i.e., the state which is the excited state in 0 + C 2~. This difference in energy ordering of 3 A' and 3 A" states results from the fact that the Px• py, Pz degeneracy of oxygen orbitals is lifted in 3 CH2leading to b1, h2. and a1 MOs whereby the lowest b 2 (a") remains doubly occupied; as a consequence, the reaction pattem between the oxygen and 3 CH 2 approach is different, which is also quite apparent in· the calculated charge transfer.