The two-state (MWC) model for cooperative oxygen binding by tetrameric (α₂β₂) hemoglobin based on concerted transitions between symmetric states (T and R) is extended to include a third, asymmetric state with one αβ dimer possessing high (R-like) oxygen affinity and the other αβ dimer possessing low (T-like) oxygen affinity. The asymmetric state is assigned a stability that corresponds to the level reported by Ackers and colleagues in the studies on mixed valence hybrids that led to their proposed “molecular code for cooperativity in hemoglobin.” However, this level of stability for the asymmetric intermediates significantly diminishes cooperativity in simulated oxygenation curves, to a degree (Hilln=2.1) that is no longer compatible with the well-established oxygenation properties of normal ferrous hemoglobin (Hilln∼3.0). Therefore, the cyanomet derivatives do not appear to be reliable analogues of intermediate oxygenation states.