Garnet-type Li5+xBaxLa3-xNb2O12 (x = 0, 0.5, 1) was prepared using a ceramic method, and H+/Li+ ion exchange was performed at room temperature using organic acids, such as CH3COOH and C6H5COOH, as proton sources. Thermogravimetric analysis showed that H+/Li+ ion exchange was nearly (100%) completed using the x = 0 member with CH3COOH, while it proceeded to about 40% for x = 0.5 and 13% for x = 1. In C6H5COOH, proton exchange proceeded to about 82% for x = 0, similar to 40% for x = 0.5, and similar to 25% for x = 1. Similar proton-exchange trends were reported in H2O, where ion exchange occurs more readily for garnets with lower Li content in Li5+xBaxLa3-xNb2O12, that is, when excess Li ions preferentially reside in the tetrahedral sites of the garnet structure.