A critical drawback of ceria-based electrolytes is that they are much more prone to reduction, thus limiting the lower bound of their electrolytic domain to much higher oxygen activity than the classic electrolyte yttria-stabilized zirconia. We examined the possibility of suppressing the partial electronic conductivity to expand the electrolytic domain width of acceptor (A(Ce)('))-doped ceria Ce(1-x)A(x)O(2-x/2) by letting reduction-induced electrons be trapped by variable valence impurities with 3d (Ti, Mn, Fe, Co, Cu) and 4f (Eu, Yb, Sm, Tb) electrons. A few 4f elements are indeed effective in extending the lower bound of the electrolytic domain by as much as 2 orders of magnitude of oxygen activity. We report the result together with the inner working of suppressing the partial electronic conductivity of Ce(1-x)A(x)O(2-x/2).