Solid solutions in the system (Al 2 (WO 4 ) 3 ) 1 - x (Sc 2 (MoO 4 ) 3 ) x have been formed and electrically characterized. These compounds are mixed trivalent cation conducting solid electrolytes in which both Al 3 + and Sc 3 + cations are mobile. However, whether Al 3 + or Sc 3 + cations represent the main charge carrying species depends on the sample composition. In solid solutions with a low Sc 2 (MoO 4 ) 3 content (x<0.6), the Al 3 + cations are the predominant charge carriers whereas Sc 3 + cations transport the main electrical current fraction in compounds with x>0.7. In comparison to pure Al 2 (WO 4 ) 3 , the electrical conductivity is maximally increased by 22 times in (Al 2 (WO 4 ) 3 ) 0 . 4 (Sc 2 (MoO 4 ) 3 ) 0 . 6 , up to 7.7x10 - 5 S cm - 1 (600 o C) with a corresponding activation energy of 76.0 kJ/mol. The trivalent cations are clearly identified as mobile charge carriers which generate the electrical properties within these materials. Neither conductivity contributions by oxygen anions nor by electronic species were observed.