The nominal Li concentration giving a maximum room temperature Li-ion conductivity σ Li ≃3.45×10−4 Scm−1 with an E a ≃0.438eV in the system Li7−x La3Hf2−x Ta x O12 fired in an alumina crucible at 1130°C for 48h occurs in the narrow range of x =0.45±0.05. The samples were prepared by solid-state reaction and characterized by XRD, SEM, electrochemical impedance spectroscopy and 7Li MAS NMR measurements.