The stability of Al-doped Li7La3Zr2O12 in air was studied in this paper. It was found that Li7La3Zr2O12 (LLZO) was unstable in humid air. Water vapor strongly affected its stability relative to composition, mechanical integrity, ionic conductivity and cell performance. X-ray diffraction studies of the surface of a one-year-old sample showed formation of La(OH)3 and Li(OH)·H2O impurity phases, indicating the decomposition of Li7La3Zr2O12. The mechanical strength of the pellet was low, and it easily crumbled. Facile H+/Li+ ion exchange in the electrolyte was confirmed by measuring the pH of an LLZO powder/water suspension. SEM observations show the reaction with water vapor initially occurs at the grain boundaries. AC impedance measurements showed the overall conductivity decayed after the pellet was stored in air for one week. Protecting a Cu0.1V2O5/Li7La3Zr2O12/Li all-solid-state battery from humidity significantly improved the cell performance at room temperature.