The stability of a NASICON-type lithium ion conducting solid electrolyte, Li1+x+y Ti2−x Al x P3−y Si y O12 (LTAP), in acetic acid and formic acid solutions was examined. XRD patterns of the LTAP powders immersed in 100% acetic acid and formic acid at 50°C for 4 months showed no change as compared to the pristine LTAP. However, the electrical conductivity of LTAP drastically decreased. On the other hand, no significant electrical conductivity change of LTAP immersed in lithium formate saturated formic acid–water solution was observed, and the electrical conductivity of LTAP immersed in lithium acetate saturated acetic acid–water increased. Cyclic voltammogram tests suggested that acetic acid was stable up to a high potential, but formic acid decomposed under the decomposition potential of water. The acetic acid solution was considered to be a candidate for the active material in the air electrode of lithium–air rechargeable batteries. The cell reaction was considered as 2Li+2 CH3COOH+1/2O2 =2CH3COOLi+H2O. The energy density of this lithium–air system is calculated to be 1477Whkg−1 from the weights of Li and CH3COOH, and an observed open-circuit voltage of 3.69V.