The interest in alternative energy sources grows rapidly and demands improved materials. The cutting-edge investigations focus attention on the development and optimization of solid electrolytes for advanced energy storage. Their chemical and structural stability defines both battery performance and lifetime, yet it is studied poorly even for well-known superionic conductors such as NASICON-based compounds. In this work, we studied the Li1.3Al0.3Ti1.7(PO4)3 (LATP) stability toward water. Corresponding ceramics were synthesized in pellet form through the solid-state reaction and had been immersed in deionized water for different periods of time with subsequent electrochemical (electrochemical impedance spectroscopy), structural (powder X-ray diffraction analysis, Raman spectroscopy, computational modeling), chemical (ceramicsenergy-dispersive X-ray spectroscopy; mother-solutionsinductively coupled plasma mass spectrometry), and morphological (scanning and transmission electron microscopy) analyses. Water exposure triggers drastic conductivity losses (64% for σt) with accompanying lithium elution (exceeds 13 atomic%) and unit cell shrinkage. All these changes reach a plateau after 2 h of water exposure.