Electrical conductivity of Ce1-yGdyO2-y/2-x (y = 0.1, 0.2), as a function of temperature and oxygen partial pressure, is measured with a complex impedance method: The Increase of electrical conductivity with reducing oxygen partial pressure can be described well by a model that assumes constant mobility of both oxygen vacancies and electrons, based on an ideal-solution model of nonstoichiometry of Gd-doped ceria. Ionic conductivity is calculated, and its activation energy is discussed. Electronic conductivity is discussed also.