A new type of lithium ion conducting solid electrolyte, LiNO3-mixed (Gd0.9La0.1)(2)O-3, was successfully developed by introducing LiNO3 into the interstitial open space in the (Gd0.9La0.1)(2)O-3 crystal lattice and the highest lithium ion conductivity of 2.02 x 10(-1) S . cm(-1) at 773 K was realized for 0.325(Gd0.9La0.1)(2)O-3-0.35LiNO(3), which was comparable to that of the highest one among LISICON-type materials. The Li+ ion conduction in the 0.325(Gd0.9La0.1)(2)O-3-0.35LiNO(3) solid was directly identified by dc electrolysis. (C) 2004 Elsevier B.V. All rights reserved.