Current pulse oximeters typically overestimate arterial oxygenation in patients with elevated levels of carbon monoxide bonded to hemoglobin, carboxyhemoglobin (COHb). The amount of overestimation is approximately equal to the amount of COHb present. Accurate measurement of arterial oxygen saturation in patients with elevated COHb can only be performed by analyzing arterial blood for COHb. Mathematical model equations for O2 and CO2 saturations of hemoglobin, S(HbO2) and S(HbCO2), are developed from the equilibrium binding of O2 and CO2 with hemoglobin.