In this study, we report the synthesis, chemical stability, and Li ion conductivity of garnet-type Li5+xBaxLa3-xTa2O12 (x = 0, 0.5, 1) in water and discuss the chemical stability in comparison to the corresponding Nb analogue. Solid-state (ceramic) synthesis method was used to prepare the investigated compounds which exhibited a cubic garnet-type structure when sintered at 950 degrees C in air. The ionic conductivity increases with increasing Li and Ba content in Li5+xBaxLa3-xTa2O12, and the x = 1 member shows the highest bulk conductivity of about 10(-4) Scm(-1) at room temperature and the lowest activation energy of 0.39 eV (25 degrees C-125 degrees C). The Ta-based Li-stuffed Li5+xBaxLa3-xTa2O12 garnets show better chemical stability in water compared to the corresponding Nb analogue, as supported by thermo-gravimetric analysis (TGA), and variable temperature powder X-ray diffraction (VT-PXRD). The proton-exchange was found to decrease with increasing Li content in Li5+xBaxLa3-xTa2O12 in water, similar to the trend in the Nb series, Li5+xBaxLa3-xNb2O12. The slightly improved chemical stability of the Ta garnets (x = 0) in moisture may be due to the lower electronegativity of Ta compared to Nb, resulting in greater covalency of the Ta-O bonds, making them more difficult to break. (C) 2013 Elsevier B.V. All rights reserved.