In the context of the ODC algorithm of Siggaard-Andersen the effects of pH, carbon dioxide tension (pCO2) and concentrations of dyshemoglobins: Carboxyhemoglobin (x COHb), methemoglobin (x MetHb) and fetal hemoglobin (x HbF) in terms of five constants, a1(0) to a5(0), were determined by a non-linear least squares analysis of 11700 records of data containing measured values of oxygen tension (pO2), oxygen saturation (sO2), as well as pH, pCO2, x COHb, x MetHb, x HbF. The standard values of a1(0) to a3(0) used in the ODC were found to be significantly different from those giving the best fit to the present data whereas the presently used coefficient for methemoglobin was found to be acceptable. The coefficient for fetal hemoglobin, a5(0), could not be determined due to insufficient data. pH and pCO2 effects were found to be correlated in the data and this fact accounts for some of the deviations of a1(0) and a2(0) from the standard values. The analysis evaluated the effect of pH as being stronger and the effect of pCO2 as being weaker than assumed in the standard model. Finally the effects of the concentrations of met- and carboxyhemoglobin were found to be correlated but no explanation of this fact is known at present.