Combinations of two or four trivalent oxides were substituted for gadolinia up to 5 mol% in the 20 mol% gadolinia-doped ceria electrolyte as additional dopants. Most of them increased the electrical conductivity of the electrolyte but some did not. Among them (YLa)0.01Gd0.19Ce0.8O1.9 and (YSm)0.03Gd0.17Ce0.8O1.9 showed the highest electrical conductivities, respectively, at 800 ℃ and 600 ℃. These experimental results were analyzed in terms of activation enthalpy and entropy for ionic conduction. Among the various possible contributions to the activation entropy, only the configurational entropy was taken into account for the explanation.