High pressure induces an irreversible aggregation of the ferrous deoxy α-chains, whereas the AHSP/α-Hb complex shows reversible hexacoordination of the α-Hb without protein aggregation. Upon pressure release, the relaxation kinetics of the transition from the hexacoordinated to pentacoordinated form of α-Hb in the presence of AHSP exhibit a biphasic shape. High pressure did not induce dissociation of α-Hb from its chaperone, as evidenced by the ligand binding kinetics that show a unique rate for the AHSP/α-Hb complex. For both free α-Hb and the AHSP/α-Hb complex, the bimolecular rate constant of CO binding (k^CO_on) versus pressure exhibits a bell shape, attributed to the transition of the rate-determining step from the chemical barrier to the migration of CO within the protein matrix.