A digital computer procedure for the conversion of oxygen tension to saturation at various temperatures, carbon dioxide tensions, and hydrogen ion concentrations is described. This program is based only on a mathematical model of the dissociation curve, which does not take into account the theory of the combination of oxygen with hemoglobin, but is describing exactly the dissociation curve proposed by Severinghaus. The use of the program is entirely pragmatic and is ready for inclusion in larger programs without modification