Electrochemical oxygen meters at Kernforschungszentrum Karlsruhe, Westinghouse Electric Company, and General Electric Company, with different reference systems (gold-air; platinum-air; and indium-In2O3) were tested and calibrated in static tests as well as in a sodium loop. The calibration in static tests was based on oxygen activities of the sodium-chromium-NaCrO2 system. In the loop the meters were calibrated on cold-trap temperatures and checked by chemical analyses of the residue of distilled sodium samples. Fair agreement has been found between the calibration in sodium, the oxygen activity of which was fixed by the chemical equilibrium of chromite formation, and the calibration based on the cold-trap temperature measured in the sodium loop. The meters with indium-In2O3 reference gave results in better agreement to theory at 550 and 500°C than the meters with air reference systems. The oxygen activity in sodium in equilibrium with NaCrO2 is very low; the calibration of oxygen meters based on this reaction gives a chance to apply the probes in the range of their highest sensitivity in highly purified sodium.