The theory of “closed space” CO 2 equilibration is studied as a basis for micro- and ultramicromethods for the determination of the acid-base status of blood. Anaerobic plasma pH before and after equilibration with CO 2 is determined by colorimetry. A micromethod using a sample of 60–100 μl of plasma is described. It is not subject to the different sources of error which can affect the electrometric methods in use at present.