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1. Spectrophotometric measurements, using a rapid mixing and stopped flow technique, have been made of the rate at which CO displaces O₂ from its combination with haemoglobin.

2. In haemoglobin solutions, buffered at pH 7·2 and 9·1, the reaction proceeds by a unimolecular dissociation as proposed by Gibson & Roughton (1955). In a Ringer—Locke solution, equilibrated with a P_CO2 of 3 cmHg and at pH 7·4, the reaction of HbO₂ with CO is a two-stage process, with a transition from one form of Hb₄O₆ to another.

3. An investigation of the reaction between CO and HbO₂ in erythrocytes, suspended in Ringer—Locke solution, indicates that the rate is determined by the chemical reaction and this also is a two-stage process.

4. The transition is probably associated with the reaction of CO₂ with Hb₄O₆, following the dissociation from fully saturated oxyhaemoglobin of an oxygen molecule. It alters the relative velocity constants of the reactions of O₂ and CO with Hb₄O₆ by 100:1.

5. The implications of these proposals of the equilibria of haemoglobin with CO and O₂ are discussed. The difference between the sigmoid equilibria curves at high HbO₂ and HbCO values can be explained as due to the different reaction pathways.