Oxides with the nominal chemical compositions Li5La3Sb2O12 and Li6SrLa2Sb2O12 were prepared by solid-state reaction. The structures were refined by the Rietveld method using powder X-ray diffraction data. The synthesis of Li5La3Sb2O12 resulted in the well known gamet-related structure plus 5 wt.% of La2LiSbO6 in the bulk. In contrast to that, Li6SrLa2Sb2O12 Could be synthesised in single gannet-related type phase. Lithium ion conductivities of Li5La3Sb2O12 and Li6SrLa2Sb2O12 were studied by the ac impedance method. The grain-boundary contribution to the total (bulk + grain-boundary) resistance is very small and about 5 and 3% for Li5La3Sb2O12 and Li6SrLa2Sb2O12, respectively, at 24 degrees C and decreases further with increase in temperature. Among the investigated compounds, Li5La3Sb2O12 exhibits the highest total (bulk + grain-boundary) and bulk ionic conductivity of 7.8 x 10(-6) and 8.2 x 10(-6) S cm(-1), respectively, at 24 degrees C. The structural data indicate that the coupled substitution Li + Sr double right arrow La leads to a closure of the bottle neck like O-O distances of the shared edges of neighbouring U octahedra and therefore reduces the mobility of Li ions in Li6SrLa2Sb2O12. Scanning electron microscope (SEM) images of the Li6SrLa2Sb2O12 compound revealed well crystallised large homogeneous grains (similar to 4.8 mu m) and the grains were in good contact with the neighbouring grain, which leads to a smaller grain-boundary contribution to the total resistance. (C) 2007 Elsevier Ltd. All rights reserved.