(Ce0.83Sm0.17)(1-x)ZrxO2-delta (x = 0 to 0.50) Solid solutions were synthesized for the first time by the hydrothermal method. The electrical properties of the solid solutions have been studied in air and under reducing conditions. Solid solutions with the fluorite structure were formed in all of the studied range of ZrO2 substitution after calcination at 1500 degreesC. With increasing ZrO2 substitution up to 30 mol %, the electronic conductivity increases under a reducing atmosphere. The (Ce0.83Sm0.17)(0.7)Zr0.3O2-delta solid solution has good mixed electronic and ionic conductivity; the total conductivity is 0.42 S/cm at p(O2) = 5.7 x 10(-21) atm and 700 degreesC with an estimated ionic conductivity of ca. 10(-2) S/cm.