It has been reported that hybridization of the equimolar mixture of cyanomethemoglobin and deoxyhemoglobin through dimer exchange reaction results in establishment of an approximately binomial (1:2:1) equilibrium distribution of these parental hemoglobins and their hybrid molecule, (alpha+CN-beta+CN-)(alpha beta), within several days under anaerobic conditions at pH 7.4, 21.5 degrees C, leading to a hyper (i.e., about 170 times) thermodynamic stability of (alpha+CN-beta+CN-)(alpha beta) relative to the stability of the other diliganded species at pH 7.4, 21.5 degrees C [Daugherty, M. A., Shea, M. A., Johnson, J. A., LiCata, V. J., Turner, G. J., & Ackers, G. K. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 1110-1114]. To examine whether the published "binomiality" for this deoxy-cyanomet hybrid system really reflects the thermodynamic stability of (alpha+CN-beta+CN-)(alpha beta), we used a binomial (1:2:1) equilibrium distribution of the equimolar mixture of cyanomethemoglobin and fully oxygenated hemoglobin as a starting condition, and then this system was rapidly deoxygenated. We found that the relative population of the hybrid was reduced to 8.6% of the total upon deoxygenation. It was also found that the hybridization experiment under anaerobic conditions was not allowed to continue for a long time due to a valency exchange reaction between deoxy and cyanomet derivatives. For instance, a 48 h incubation resulted in the oxidation of 44% of Fe2+ to Fe3+ hemes in the original deoxyhemoglobin and the reduction of 42% of Fe3+ to Fe2+ hemes in the original cyanomethemoglobin. These results suggest that a real distribution of the deoxy-cyanomet hybrid system at equilibrium is fairly far from 1:2:1 (binomial distribution), and the thermodynamic stability of (alpha+CN-beta+CN-)(alpha beta) is less than one-tenth of the hyperstability previously reported. In addition, most of the previous results on deoxy-cyanomet valency hybrids placed under long anaerobic conditions should be subject to reexamination due to possible valency exchange reactions.