Abstract: The invention relates to an apparatus for automatically determining the oxygen dissociation curve (ODC) of whole blood or of a hemoglobin solution. Blood or hemoglobin having a predetermined oxygen saturation level are admixed with deoxygenated blood or a hemoglobin solution and the oxygen partial pressure of the mixture, while being formed, is measured. The output of the P.sub.O.sbsb.2 electrode, in the form of an electrical signal, is fed into a computer and two functions of the P.sub.O.sbsb.2 (saturation and total oxygen) are calculated, giving the required curve by means of recording and/or displaying means. In this way, the ODC can be determined for whole blood under near physiological conditions of pH, hemoglobin concentration, hematocrit, temperature, P.sub.CO.sbsb.2, and organic phosphates content.