Oxide ion electrolytes determine the temperature of operation of solid oxide fuel cells, oxygen separation membranes, and oxygen sensors. There is a strong incentive to lower their operating temperatures, in a solid oxide fuel cell, for example, from T-op > 800 degrees C to T-op approximate to 500 degrees C. The use of low-cost Na+ rather than K+ as the dopant in monoclinic SrSiO3 (C12/C1) is shown to provide a larger solid solution range (0 < x <= 0.45) in Sr1-xNaxSiO3-0.5x and to achieve an oxide ion conductivity sigma(o) >= 10(-2) S.cm(-1) by 525 degrees C as a result of lowering the temperature of a smooth transition to full disorder of the mobile oxide ions. The Sr1-xNaxSiO3-0.5x electrolytes are much less hygroscopic than Sr1-xNaxSiO3-0.5x and are stable with a nickel composite anode in 5% H-2/Ar as well as with cathodes such as La1-xSrxMnO3-delta and Sr0.7Y0.3CoO3-delta in air, which makes them candidate electrolytes for intermediate-temperature solid oxide fuel cells or for other applications of oxide ion electrolytes.