Garnet-type electrolytes are currently receiving much attention for applications in Li-ion batteries, as they possess high ionic conductivity and chemical stability. Doping the garnet structure has proved to be a good way to improve the Li ion conductivity and stability. The present study includes effects of Y- doping in Li5La3Nb2O12 on Li ion conductivity and stability of “Li5+2xLa3Nb2-xYxO12” (0.05 ≤ x ≤ 0.75) under various environments, as well as chemical stability studies of Li5+xBaxLa3-xM2O12 (M = Nb, Ta) in water. “Li6.5La3Nb1.25Y0.75O12” showed a very high ionic conductivity of 2.7 х 10−4 Scm−1 at 25 °C, which is comparable to the highest value reported for garnet-type compounds, e.g., Li7La3Zr2O12. The selected members show very good stability against high temperatures, water, Li battery cathode Li2CoMn3O8 and carbon. The Li5+xBaxLa3-xNb2O12 garnets have shown to readily undergo an ion-exchange (proton) reaction under water treatment at room temperature; however, the Ta-based garnet appears to exhibit considerably higher stability under the same conditions.