The stability of the high lithium ion conducting glass ceramics, Li1+x+yTi2-xAlxSiyP3-yO12 (LTAP) in alkaline aqueous solutions with and without LiCl has been examined. A significant conductivity decrease of the LTAP plate immersed in 0.057 M LiOH aqueous solution at 50 degrees C for 3 weeks was observed. However, no conductivity change of the LTAP plate immersed in LiCl saturated LiOH aqueous solutions at 50 degrees C for 3 weeks was observed. The pH value of the LiCl-LiOH-H2O solution with saturated LiCl was in a range of 7-9. The molarity of LiOH and LiCl in the LiOH and LiCl saturated aqueous solution were estimated to be 5.12 and 11.57 M, respectively, by analysis of Li+ and OH-. The high concentration of LiOH and the low pH value of 8.14 in this solution suggested that the dissociation of LiON into Li+ and OH- is too low in the solution with a high concentration of Li+. These results suggest that the water stable LTAP could be used as a protect layer of the lithium metal anode in the lithium/air cell with LiCl saturated aqueous solution as the electrolyte, because the content of OH- ions in the LiCl saturated aqueous solution does not increase via the cell reaction of Li + 1/2O(2) + H2O -> 2LiOH, and LTAP is stable under a deep discharge state. (C) 2011 Elsevier B.V. All rights reserved.