The chemical and electrochemical stability of high-purity lithium aluminum titanium phosphate (LATP) as a solid state electrolyte and physical separator in aqueous electrochemical cells was evaluated using LiOH, LiCl, LiNO3, and LiCOOCH3 salts as the Li source. LATP, with formula Li1.3Al0.3Ti1.7P3O12, was found to be most stable between pH 8–9, with the longest cell operating continuously at 25 mA cm−2 for 625 h at 40 °C in LiCOOCH3. Biaxial strength was 191 ± 11 MPa when tested in mineral oil, 144 ± 13 MPa as measured in air, and 26 ± 7 MPa after exposure to deionized water, suggesting that LATP undergoes stress-corrosion cracking. After exposure to LiOH, the strength was 76 ± 19 MPa. The decrease in strength was observed despite there being no measurable change in a.c. impedance spectra, x-ray diffraction, or sample mass, suggesting phosphate glasses at grain boundaries. At pH values outside of the 7–10 range, eventual membrane degradation was observed in all aqueous systems under electrochemical conditions. While LATP was surprisingly resistant to static corrosion in a hot, aqueous LiOH solution, electrochemical degradation was observed at the cathode due to subsurface pitting. Strength measurements were more instructive than impedance measurements in detecting this degradation.