By employing an electrochemical technique incorporating magnesia-stabilized zirconia electrolyte, the activities of iron oxide, Fe x O, were measured in magnesiowüstite, Fe x O-MgO solid solution, in equilibrium with solid metallic iron at temperatures between 1373 and 1573 K. The sub-regular solution model was applied to the Fe x O activities, and the calculated Fe x O activities were in good agreement with the experimental values in this study and the literatures at temperatures between 1073 and 1573 K with an accuracy of AE0:02.