Garnet-structure related metal oxides with the nominal chemical composition of Li 5 La 3 Nb 2 O 12 , In-substituted Li 5.5 La 3 Nb 1.75 In 0.25 O 12 and K-substituted Li 5.5 La 2.75 K 0.25 Nb 2 O 12 were prepared by solid-state reactions at 900, 950, and 1000 1C using appropriate amounts of corresponding metal oxides, nitrates and carbonates. The powder XRD data reveal that the In-and K-doped compounds are isostructural with the parent compound Li 5 La 3 Nb 2 O 12 . The variation in the cubic lattice parameter was found to change with the size of the dopant ions, for example, substitution of larger In 3+ (r CN6 : 0.79 A Λ) for smaller Nb 5+ (r CN6 : 0.64 A Λ) shows an increase in the lattice parameter from 12.8005(9) to 12.826(1) A Λat 1000 1C. Samples prepared at higher temperatures (950, 1000 1C) show mainly bulk lithium ion conductivity in contrast to those synthesized at lower temperatures (900 1C). The activation energies for the ionic conductivities are comparable for all samples. Partial substitution of K + for La 3+ and In 3+ for Nb 5+ in Li 5 La 3 Nb 2 O 12 exhibits slightly higher ionic conductivity than that of the parent compound over the investigated temperature regime 25-300 1C. Among the compounds investigated, the In-substituted Li 5.5 La 3 Nb 1.75 In 0.25 O 12 exhibits the highest bulk lithium ion conductivity of 1.8 Γ 10 Γ4 S/cm at 50 1C with an activation energy of 0.51 eV. The diffusivity (''component diffusion coefficient'') obtained from the AC conductivity and powder XRD data falls in the range 10 Γ10 -10 Γ7 cm 2 /s over the temperature regime 50-200 1C, which is extraordinarily high and comparable with liquids. Substitution of Al, Co, and Ni for Nb in Li 5 La 3 Nb 2 O 12 was found to be unsuccessful under the investigated conditions.