Cubic, fluorite-structured solid solution's Zr1-xYxO2-x/2 (YSZ; x = 0.4-0.7) were prepared by sol-gel synthesis. Impedance measurements on pellets of 85% approximate density sintered at 1300 degrees C for 24 h showed strong evidence of oxide ion conduction with an inclined Warburg spike at low frequencies and capacitance values of similar to 10(-6) F cm(-1) at 40 Hz. Arrhenius plots of total pellet conductivities were linear with activation energies of 1.4-1.56 eV. The conductivity decreased with x and was 2-4 orders of magnitude lower than that with optimized YSZ, x = 0.08. When the atmosphere was changed from N-2 to O-2 during impedance measurements, two reversible effects were seen: the Warburg spike contracted greatly, and the sample resistance decreased. These effects were more noticeable at higher x and are attributed to the introduction of p-type electronic conduction, in parallel with the preexisting oxide ion conduction. A similar reversible result was observed upon application of a direct-current (dc) bias during impedance measurements. When either pO(2) is increased or a dc bias is applied, hole creation is believed to arise by the ionization Of underbonded oxide ions situated near the Y3+ dopant ions. The ionized electrons are trapped at surface oxygen species, and the holes that are left on oxygen are responsible for p-type conduction. The electrolytic domain of x extends up to approximately 10(-2) atm of O-2 before p-type conduction is observed. The Upper pO(2), limit of the electrolytic domain of x = 0.08 is not known but is likely to be dose to or slightly above 1 atrn of O-2.