The stability of the high lithium ion conducting glass ceramics, Li1+x+y Ti2−x Al x Si y P3−y O12 (LTAP) in alkaline aqueous solutions with and without LiCl has been examined. A significant conductivity decrease of the LTAP plate immersed in 0.057M LiOH aqueous solution at 50°C for 3 weeks was observed. However, no conductivity change of the LTAP plate immersed in LiCl saturated LiOH aqueous solutions at 50°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 11.57M, 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 LiOH 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+1/2O2 +H2O→2LiOH, and LTAP is stable under a deep discharge state.