We use large-scale multiconfiguration self-consistent field/configuration interaction calculations to characterize the predissociation mechanism of the (B) over tilde (2)A' state of HCO through conical intersection with the (X) over tilde (2)A' ground state. We locate two regions of intersection: the first represents a highly bent HCO that is 8 kcal/mol energetically lower than the (B) over tilde state minimum, with a barrier height of 26 kcal/mol. Energy points on the (B) over tilde (2)A' potential surface connecting these extrema were also calculated. This region emphatically illustrates the feasibility of a nonradiative decay mechanism consistent with latest experimental findings of purely vibronic coupling mechanism. The second region of intersection represents a confluence of three linear (2)Sigma(+)-(2)Pi states crossings, 53 kcal/mol below the (B) over tilde state minimum. A barrier about 21 kcal/mol above the (B) over tilde state equilibrium structure is located and assigned to the entrance channel of H-CO((3)Pi).