Abstract: The present invention relates to an oxygen ion conductor consisting of a composition consisting of (1-x-y)ZrO.sub.2 --xSc.sub.2 O.sub.3 --yAl.sub.2 O.sub.3, and to solid fuel cells employing this ion conductor as the solid electrolyte. In order to obtain more desireable characteristics in the aforementioned composition, it is preferrable to apply limits to the composition such that: 0.05<x+y.ltoreq.0.16 and x>0, y>0, or, alternatively, such that: 0.07.ltoreq.x+.ltoreq.0.13 and 0.005.ltoreq.y.ltoreq.0.02. In the present invention phase transitions occuring during temperature change from room temperature to high temperatures are eliminated and mechanical strength with respect to the heat cycle is increased by adding as a secondary dopant a specific quantity of Al.sub.2 O.sub.3 to a specific ZrO.sub.2 --Sc.sub.2 O.sub.3 system composition which has a high degree of ion conductivity.