Hydrogen and oxygen behaviors in a sodium test loop similar to the sodium-cooled fast breeder reactor were examined. Hydrogen partial pressure equilibrated to the sodium was measured by a membrane-permeation-type hydrogen-meter. An electrochemical oxygen-meter was used to determine the oxygen activity. The measured hydrogen pressure was converted to the hydrogen concentration through the Sieverts’ constant, and the oxygen activity was also converted to the oxygen concentration through the characteristics of the oxygen-meter cells. Hydrogen and oxygen solubilities in liquid sodium were obtained as functions of the cold-trap temperature from 115 to 208°C. Both data have a similar temperature dependence and were anomalously larger than the extrapolated values in the temperature region lower than 180°C. At 300°C, the addition of hydrogen to the liquid sodium, valved off from the purification apparatus, lowered the oxygen activity in the liquid sodium. The experimental results suggest a significant interaction between hydrogen and oxygen and the existence of (OH) species in liquid sodium. Using the solubility data of oxygen, hydrogen, and (OH) species, the free energy change of the reaction,(O)in Na + (H) in Na = (OH)in Na,was obtained and was ΔG°(kcal/mole) = −28 + 0.043 T. From the temperature dependence of the equilibrium hydrogen pressure, the apparent heat of a solution of hydrogen in liquid sodium was obtained as 1.6 kcal/mole.