Novel compositions of Nd3+ and Dy3+ codoped cerium oxide according to the system of Ce0.8Ndx–yDyyO2–{x–y/2+y} (x = 0.2; y = 0.04, 0.08, 0.1) have been synthesized by a simple sol–gel method and studied as electrolytes for intermediate-temperature solid oxide fuel cells (IT-SOFCs). Thermal, microstructural, optical, and electrical properties have been enhanced to different extents by the addition of Nd3+ and Dy3+ ions in cerium lattice, in particularly Dy3+ ions because of their low ionic radius mismatch. Thermogravimetric/differential scanning calorimetric (TG/DSC) analysis exhibited a small weight loss and high thermal stability in the intermediate temperature range (400–800 °C). The addition of Dy3+ ions stabilized the cubic fluorite structure, which is confirmed from X-ray diffraction (XRD) studies. Lattice parameter expansion and contraction have been observed on account of their ionic radii trend. The formation of cubic fluorite structure has been confirmed by high-resolution transmission electron mi...