<p>Zirconia oxygen sensors are used in the steel-refining process to instantly measure the oxygen concentration in molten steel. It is desirable to develop sensors that are capable of continuous and long-time measurements, but such measurements are difficult with conventional sensors. In this study, the factors that hinder long-time measurement by a single zirconia oxygen sensor with a mixture of Cr and Cr₂O₃ as the reference electrode were investigated. Continuous measurements of a molten iron containing carbon and aluminum for deoxidization were carried out with the sensor. The electromotive force between the reference electrode (positive) and sample electrode (negative) of the sensor decreased from a positive value to zero with time. This was because reduction of chromium oxide at the surface of the zirconia solid electrolyte was promoted owing to oxygen diffusion from the inside to the outside of the zirconia tube and the equilibrium in the reference electrode was disturbed during the measurements. In addition, the reference electrode at the surface partially and temporarily melted owing to diffusion of the components from the electrolyte to the reference electrode. Additionally, a test to extend the lifetime of the sensor was performed with application of a direct electrical current to the sensor. The electromotive force with application of a current was maintained at a higher value than that without application of a current owing to promotion of re-oxidation at the zirconia surface. However, it was found that a high current can cause over-oxidation and dissolution of the reference electrode at the surface.</p>