CeO ₂-based solid solutions with a fluorite structure are promising materials as electrolytes of medium-temperature electrochemical devices. This work presents the results of systematic studies of structural and electric properties and oxygen nonstoichiometry of the Ce _0.8(Sm _1-xCa _x) _0.2O _2-δ system in a wide range of concentrations of 0 < x < 1 performed in order to establish the causes affecting the system conductivity and its behavior in a reducing medium. It is found that a single-phase solid solution of the fluorite type is formed in the whole concentration range. Parameters of its lattice cells decrease linearly at an increase in the concentration of Ca ²⁺. Conductivity in air grows when calcium is added due to a decrease in the grain boundary resistance. The maximum conductivity in air was obtained for the composition of Ce _0.8(Sm _0.8Ca _0.2) _0.2O _2-δ and is 13.71 × 10 ^-3 S/cm at 873 K. Studies of the dependence of conductivity of the partial pressure of oxygen showed that electron conductivity is observed at a higher oxygen partial pressure at an increase in the temperature and calcium concentration. The critical partial pressure of oxygen (PO* ₂) for the compositions of Ce _0.8(Sm _1-xCa _x) _0.2O _2-δ with x = 0; 0.2, and 0.5 is 1.83 × 10 ^-16, 1.73 × 10 ^-13, and 3.63 × 10 ^-13 atm at 1173 K, respectively, and 2.76 × 10 ^-21, 5.05 × 10 ^-18, and 1.31 × 10 ^-18 atm at 1023 K.