It has been suggested (J. Am. Chem. Sec. 1993, 115, 6904) that CH(a(4) Sigma(-)) is collisionally quenched to CH-(X(2) Pi) by CO(X(1) Sigma(+)) via an intemlediate complex involving the 4A " state of the ketenyl (HCCO) radical. To shed further light on this proposition, points of the surface of intersection of the (4)A " potential energy surface with the (2)A’ and (2)A " potential energy surfaces, surfaces correlating with CH(X(2) Pi) + CO(X(1) Sigma(+)), were determined. The (4)A "-((2)A’, (2)A ") spin-orbit interactions were determined and analyzed in a basis that simplifies the incorporation of Kamers’ degeneracy. The noncrossing rule for odd electron states in the presence of the spin-orbit interaction is discussed. The computational results:support a generalization of the proposed mechanism of CH(a(4) Sigma(-)) quenching.