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Codoping is used to enhance the ionic conductivity of ceria-based electrolytes. Sm 3+ and Nd 3+ were selected as codopants to promote low migration energy paths for oxygen vacancy diffusion, thereby increasing the ionic conductivity. Moreover, the use of codopants also increases the pre-exponential factor in the Arrhenius relationship, thus further improving the ionic conductivity. The ionic conductivity of Sm x Nd x Ce 1− 2 x O 2− δ solid solutions is measured using electrochemical impedance spectroscopy. It was observed that for Sm 0.075 Nd 0.075 Ce 0.85 O 2− δ⁠, the grain ionic conductivity was 14.0× 10− 3 S cm− 1 at 550 C⁠, which makes it one of the most promising ceria-based electrolytes for intermediate temperature solid oxide fuel cells.