Co-doped ceria electrolytes of the Ce0.80Nd0.2-xSmxO1.(90) (x = 0.05, 0.10, and 0.20) powders were prepared with the Pechini method. The aim of the present investigation was to study the effect of co-doping on the ionic conductivity of ceria (CeO2) for its use as solid electrolyte in intermediate temperature solid oxide fuel cells. Results of the X-ray diffraction analyses showed that all powders calcined at 600 degrees C for 4 h were single phase with cubic fluorite structure. The average crystallite sizes calculated by the Scherrer formula were found as between 24 and 30 nm. The samples were sintered at 1400 degrees C for 6 h to obtain dense ceramics (over 93%). The two-probe a.c. impedance spectroscopy was used to study the ionic conductivity of the co-doped ceria samples. Effects of co-doping on crystal structure and electrical conductivity were studied comparing with the single doping. The results indicated that Ce0.80Nd0.10Sm0.10O1.(90) had the highest electrical conductivity, sigma(750) degrees C which is equal to 4.42 x 10(2) S/cm. It can be therefore concluded that co-doping with Sm-3 vertical bar and Nd-3 vertical bar cations can further improve the electrical properties of ceria electrolytes compared to single element doped ceria (Ce0.80Sm0.20O1.(90)).