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We demonstrate for the first time the instability of the lithium garnet Li 7 La 3 Sn 2 O 12 toward moisture. In ambient air a spontaneous Li þ /H þ exchange occurs leading to Li 7-x H x La 3 Sn 2 O 12 . The lithium released is combined with the atmospheric CO 2 under the Li 2 CO 3 form making Li 7 La 3 Sn 2 O 12 a CO 2 absorbent. The structural study of the exchanged Li 7-x H x La 3 Sn 2 O 12 phase has been performed on samples obtained by heating at reflux the mother compound in a solution of benzoic acid/ ethanol during one week. After chemical analysis of the lithium content and thermal analysis, the compound has been structurally characterized by the Rietveld method from powder and single crystal XRD experiments. It was found to crystallize in the cubic space group Ia3d with a Li þ distribution comparable to that observed in other cubic lithium garnets: three lithium sites partially occupied (24d, 48g, and 96h). The total ionic conduction of the exchanged garnet has been studied by impedance spectroscopy and compared to that of the mother form Li 7 La 3 Sn 2 O 12 . This study revealed that after Li þ /H þ exchange the conduction remains unchanged.