We have investigated the functional properties of hemoglobin (Hb) valency hybrids, and specifically whether it makes a difference if the oxidized subunits are on the same dimer (asymmetric hybrid) or not. CO recombination kinetics were used to probe the allosteric equilibrium of tetramers with two oxidized subunits. Asymmetric hybrids were prepared by mixing HbCO with a large excess of cyano-metHb; dimer exchange occurs in the order of seconds, producing a population of fully liganded [dimer-CO/dimer-CN] hybrids; these hybrids could then be photolysed to study CO rebinding to the doubly liganded [dimer-deoxy/dimer-CN] species. Before mixing, the HbCO samples typically show 30 to 50% slow phase, characteristic of deoxy (or T-state) Hb. Addition of HbCN to these samples decreased the slow fraction. The higher the ratio of HbCN to HbCO, the less slow phase was observed, with about 5% slow phase at a ratio of 10:1. This would indicate that the photoproduct [deoxy-dimer/ dimer-CN] is not predominantly in the low-affinity (T-state) conformation. We did not observe the difference between asymmetric and symmetric hybrids expected from published studies. The difference between the present flash photolysis results and the published equilibrium studies could be due to a kinetic factor: if the conversion to the T-state is slow after photolysis, then the biomolecular kinetics will reflect the pre-flash conditions of fully liganded (R-state) hemoglobin.