Abstract The trivalent ionic conduction in the Sc 2(WO 4) 3–Lu 2(WO 4) 3 solid solution, which possesses the Sc 2(WO 4) 3-type structure was investigated. The Sc site substitution in Sc 2(WO 4) 3 for larger Lu 3+ occurred completely and the cell volume of the solid solution linearly expanded with increasing the Lu content. The ionic conductivity of the solid solution system enhanced with increasing the Lu substitution of the Sc site in Sc 2(WO 4) 3 up to 50 mol%. On the other hand, the conductivity decreased by replacing the Lu site in Lu 2(WO 4) 3 for Sc 3+ up to 40 mol%. A discontinuity appeared in the relation between the ionic conductivity and the composition of the solid solution around 50–60 Lu mol% substitution of the Sc site in the Sc 2(WO 4) 3-type structure. The electrolysis results derive the conclusion that the solid solution is a Sc 3+ and Lu 3+ trivalent mixed conductor and the mainly conducting species is Sc 3+ and Lu 3+ below and above 50–60 Lu mol%, respectively.