Photodissociation of the CO molecule from carbon monoxide complexes of myoglobin and hemoglobin was examined between 4.2 and 80 °K. Experimental results have demonstrated that: 1. (I)|The photodissociated form was stable below 10 °K. 2. (II)|The ratio of the quantum yield of hemoglobin to myoglobin was roughly estimated to be 0.38. 3. (III)|The recombination of dissociated CO occurred by raising the temperature. 4. (IV)|The recombination reaction of photodissociated myoglobin and hemoglobin with CO was biphasic, whereas that of cobalt myoglobin with O 2 was monophasic. 5. (V)|Activation energies for fast and slow reactions of myoglobin and hemoglobin, and for monophasic reaction of cobalt myoglobin are listed below: Fast Slow Mb(Fe 2+) + CO 124 cal/mole 124 cal/mole Hb(Fe 2+) + CO 124 cal/mole 230 cal/mole Mb(Co 2+ + O 2 570 cal/mole 6. (VI)|In the near infrared region the photodissociated forms of myoglobin and hemoglobin gave absorbance peaks at 770 nm, whereas deoxygenated forms of myoglobin and hemoglobin gave absorbance peaks at 758 and 750 nm, respectively, which suggests that the near infrared band is sensitive to the protein structure and may be called the “conformation band”. Several discussions have been held on the nature of the photodissociated form and the biphasic property of the reaction of myoglobin and hemoglobin, compared with those of cobalt myoglobin. It is now concluded that the new species, corresponding to the “rapidly reacting form” of hemoglobin by Gibson, Q. H. (1959) Biochem. J. 71, 293–303, and, more recently, of hemoglobin, single chain proteins and noncooperative aggregated forms by Alpert, B., Banerjee, R., and Lindqvist, L. (1974) Proc. Natl. Acad. Sci. U.S. 71, 558–562, is certainly trapped at 4.2 °K by photodissociating the carbon monoxide and oxygenated myoglobin and hemoglobin as well as the oxygenated cobalt myoglobin and hemoglobin.