The trivalent ionic conduction in the Sc-2(WO4)(3)-Gd-2(WO4)(3) solid solution with the Sc-2(WO4)(3)-type structure was investigated. The solubility limit of Gd3+ in Sc-2(WO4)(3) is 60 mol% in the Sc site and the lattice size of the solid solution isotropically expands with increasing the Gd3+ content. The ionic conductivity of the system greatly increased with an increase of the Gd3+ substitution and the maximum value of 6.2 x 10(-5) S cm(-1) at 600 degrees C was obtained for the composition, Sc1.6Gd0.4(WO4)(3), which is three times higher than that of pure Sc-2(WO4)(3). The electrolysis result explicitly indicates that the predominant mobile species in the solid solution with the Sc-2(WO4)(3)-structure is trivalent Sc3+ ion and that Gd3+ ion in the electrolyte acts as a lattice forming ion to obtain a suitable lattice size for the Sc3+ ion migration.