The mechanism of spin-forbidden reaction O(P-3) + HCCH((X) over tilde(1)Sigma(g)(+)) --> HCCHO(1,2(3)A) --> HCCHO((1)A) --> CH2((a) over tilde(1)A(1)) + CO(X(1)Sigma(g)(+)) is discussed. Portions of the (3)A "-(1)A＇, (3)A＇-(1)A＇, and (3)A "-(3)A＇ surfaces of intersection and the spin-orbit coupling matrix elements are determined. An intermediate complex driven model is proposed. Intersystem crossing is expected to occur over the entire region between the intermediate complex, (HCCHO)-H-3, and the transition state for the spin-allowed rearrangement, (HCCHO)-H-3 --> (H2CCO)-H-3. The barrier for the reaction arises from the formation of the adduct (HCCHO)-H-3 and is thus the same as that of the spin-allowed process forming CH2((X) over tilde(3)B(1)).