A novel, nonadiabatic reaction path for H₂ + CO molecular dissociation of formaldehyde via an extended S₁/S₀ conical intersection seam has been mapped out using the CAS-SCF method with a full valence active space (10 electrons, 9 orbitals). Two conical intersection geometries have been optimized, CsCoIn, a saddle point in the intersection space, and C1CoIn, which is the lowest-energy crossing point. A minimum-energy path connecting these points along a seam has also been characterized. In addition to the conventional and “roaming-atom” mechanismswhere internal conversion takes place before ground-state dissociationwe suggest that a strictly nonadiabatic mechanism can operate, where internal conversion and dissociation take place in concert.