(Ce0.83Sm0.17)(1-x)Ln(x)O(2-delta) (Ln = Pr, Tb, 0 less than or equal to x less than or equal to 0.1) solid electrolytes were prepared by a soft hydrothermal method at 260 degrees C. Transmission electron microscopy (TEM) and X-ray diffraction were used to clarify the structure and morphologies of the crystalline powers. The average crystallite size observed in the TEM is in the range of 10-60 nm, close to the those calculated by the Scherrer formula from X-ray patterns. The uniformly small particle size of the hydrothermally prepared materials allows sintering of the samples into highly dense ceramic pellets at 1300-1400 degrees C, a significantly lower temperature, compared to that of 1600-1650 degrees C required for samples prepared by solid-state techniques. The variation of the electrical conductivity with the partial pressure of oxygen, P-O2, in the range 0.21 to 10(-24) atm at various temperatures was used to determine the electrolytic domain boundary (EDB) of (Ce0.83Sm0.17)(1-x)(Tb/Pr)(x)O2-delta for 0 less than or equal to x less than or equal to 0.1. The EDB of Ce0.83Sm0.17O2-delta is similar to 10(-18) atm at 700 degrees C. It is shown that neither Pr nor Tb doping has a significant effect on the EDB of Ce0.83Sm0.17O2-delta.