The minimum energy path for the reaction O(P-3(g)) = C2H4((1)A(g)). has been calculated by optimizing all relevant geometrical parameters along the approach of oxygen to ethene. A barrier of 4.7 kcal/mol in the 3A"(...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 level. The corresponding values at the lower-level treatment CASSCF are 9 kcal/mol 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 lowest-energy structure (asymmetric OC2H4)is around 5 kcal/mol. The energy gap to the first excited state 3A’- (...9a’10a’3a"(2)) is found to be 3.6 kcal/mol in MRD-CI calculations at the ground-state minimum. Comparison with (CH2)-C-3 + C2H4 shows that in this system the lowest-energy surface is 3A’, i;e., the state which is the excited state in O + C2H4 This difference in energy ordering of 3A’ and’A" states results from the fact that the p(x), p(y), p(z) degeneracy of oxygen orbitals is lifted in (CH2)-C-3 leading to b(1), b(2), and a(1) MOs whereby the lowest b(2) (a") remains doubly occupied; as a consequence,the reaction pattern between the oxygen and (CH2)-C-3 approach is different, which is also quite apparent in the calculated charge transfer.