A reference method for tonometry of blood is described. The document covers the theory of tonometry, the materials and equipment needed, and essential aspects of the tonometry procedure for blood. The partial pressures of oxygen and carbon dioxide in tonometered blood are accurately known and therefore this blood is recommended for assessing the accuracy of blood gas analyzers. Tonometry of blood samples from patients may also be used in the determination of acid-base quantities and hemoglobin-oxygen affinity e.g. p50.