CeO ₂-based solid solutions with a fluorite structure are promising materials as electrolytes of medium-temperature electrochemical devices: electrolytic cells, oxygen sensors, and solid oxide fuel cells. In this work, studies are presented of the effect of the dopant cation radius and its concentration on the physico-chemical properties of the Ce _1-xLn _xO _2-δ solid solutions (x = 0-0.20; Ln = La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Yb) and also of multicomponent solid solutions of Ce _1-xLn _x/2L O _2-δ (x = 0-0.20; Ln = Sm, La, Gd and Ln' = Dy, Nd, Y) and Ce _1-x-ySm _xMyO _2-δ (M = Ca, Sr, Ba) obtained using the solid-phase synthesis technique. Electric properties of the samples were studied in the temperature range of 623-1173 K and in the oxygen partial pressure range of 0.01-10 ^-22 MPa. The values of oxygen critical pressure (PO* ₂) are presented, at which the ionic and electron conductivity values are equal. The values were calculated on the basis of experimental dependences at 1023 K at the assumption that the ionic conductivity value is determined only by the dopant concentration and its effective ionic radius and is independent of the oxygen partial pressure.