High Cs+ ion conductivity was successfully realized in solids at temperatures below 250 degrees C by doping Cs-bis(trifluoromethanesulfonyl) amide (CsTFSA) into cubic rare earth oxides of R2O3. Among the (1-x)R(2)O(3-x)CsTFSA solids prepared, a 0.75Gd(2)O(3)-0.25CsTFSA solid showed the highest Cs+ ion conductivity of 3.4 x 10(-2) S cm(-1) at 250 degrees C, which is the highest value yet reported for a solid Cs+ ion conductor.