Oxides with the nominal chemical formula Li(6)ALa(2)Ta(2)O(12) (A = Sr, Ba) have been prepared via a solid-state reaction in air using high purity La2O3, LiOH.H2O, Sr(NO3)(2), Ba(NO3)(2), and Ta2O5 and are characterized by powder X-ray diffraction (XRD) in order to identify the phase formation and AC impedance to determine the lithium ion conductivity. The powder XRD data of Li(6)ALa(2)Ta(2)O(12) show that they are isostructural with the parent garnet-like compound Li5La3Ta2O12. The cubic lattice parameter was found to increase with increasing ionic size of the alkaline earth ions (Li6SrLa2Ta2O12: 12.808(2) Angstrom; Li6BaLa2Ta2O12: 12.946(3) Angstrom). AC impedance results show that both the strontium and barium members exhibit mainly a bulk contribution with a rather small grain-boundary contribution. The ionic conductivity increases with increasing ionic radius of the alkaline earth elements. The barium Compound, Li6BaLa2Ta2O12, shows the highest ionic conductivity, 4.0 x 10(-5) S cm(-1) at 22degreesC with an activation energy of 0.40 eV, which is comparable to other lithium ion conductors, especially with the presently employed solid electrolyte lithium phosphorus oxynitride (Lipon) for all-solid-state lithium ion batteries. DC electrical measurements using lithium-ion-blocking and reversible electrodes revealed that the electronic conductivity is very small, and a high electrochemical stability (> 6 V/Li) was exhibited at room temperature. Interestingly, Li(6)ALa(2)Ta(2)O(12) was found to be chemically stable with molten metallic lithium.