The Li-ion conductor “Li6.5La3Zr1.5Ta0.5O12” (LLZT) with garnet structure is prepared by solid-state reaction in an alumina crucible; its stability in water is investigated at room temperature. A Li+/H+ exchange is characterized by TGA, 7Li and 27Al MAS NMR, neutron diffraction, and TEM. In water, Li+/H+ exchange leads to a new garnet oxide “Li6.5 − xHxLa3Zr1.5Ta0.5O12” (H-LLZT) with space group Ia–3d; the lattice parameter of H-LLZT is increased by the exchanged protons. Both LLZT and a grain boundary phase LiAlO2 are unstable in water. The exchanged protons in H-LLZT displace Li from the octahedral sites bridging the 24d tetrahedral sites; they form a strong OH bond that distorted the site. The amorphous phase LiAlO2 in the grain boundary dissolves on exposure to water, which inhibits Li-ion transport across the grain boundary.