The relative orientations of carbon monoxide (CO) bound toand photodissociated from myoglobin in solution have been determinedwith time-resolved infrared polarization spectroscopy. The bound CO isoriented < or = 7 degrees from the heme normal, corresponding tonearly linear FE-C-O. Upon dissociation from the Fe, CO becomestrapped in a docking site that orientationally constrains it to lieapproximately in the plane of the heme. Because the bound and "docked"CO are oriented in nearly orthogonal directions CO binding from thedocking site is suppressed. These solutions results help to establishhow myoglobin discriminates against CO, a controversial issuedominated by the misconception that Fe-C-O is bent.