A single phase NASICON-type lithium conducting solid electrolyte, Li1.4Ti1.6 Al-0.4(PO4)(3) (LTAP), was synthesized from the precursor prepared by a sol-gel method with citric acid. The bulk and grain boundary conductivity in open air (relative humidity; ca. 60%) of the LTAP sintered at 950 degrees C was estimated to be 1.8 x 10(-3) S cm(-1) and 1.3 x 10(-3) S cm(-1) at 25 degrees C, respectively, from the impedance profile analysis. The grain boundary resistance almost vanished following immersion in a saturated LiCl aqueous solution at 50 degrees C for one week, and the total conductivity at 25 degrees C increased from 7.5 x 10(-4) S cm(-1) to 9.2 x 10(-3) S cm(-1) in open air. The high conductivity has not changed for a long period in open air. The total conductivity of the immersed LTAP was dependent on the humidity in the atmosphere and was comparable to that of the pristine LTAP at as low humidity as in a glove box (dew point of ca. -80 degrees C). The conductivity enhancement was explained by the formation of a low resistance LiCl-xH(2)O phase trapped in the grain boundaries of LTAP nanoparticles. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.018204jes] All rights reserved.