Lithium oxide excess and powder bed method are conventionally used for synthesis of lithium-conducting solid electrolytes with cubic garnet structure. In this work Li6.75La3Zr1.75Nb0.25O12 garnet was sintered at 1200 °C on alumina and zirconia supports with controlled amount of lithium oxide excess. It was found that the powder bed method did not prevent garnet pellets from alumina contamination. The quantity of aluminum penetrated into samples correlated with the amount of lithium oxide excess added and varied from 0.2 to 0.5 wt%, as indicated by ICP AES. In contrast to alumina, zirconia did not affect chemical composition of the pellets. Taking this into account to avoid any alumina contamination a modified powder bed method was developed. However, non-contaminated samples showed lower density after sintering when compared to contaminated ones. Lithium oxide excess in the samples is likely to react with the alumina with formation of eutectic that melts at 1200 °C. The liquid eutectic phase acts as a sintering aid and promotes densification up to relative densities of 96%. Electrical properties of the pellets were measured by EIS technique. Bulk conductivity of the dense pellets was 5 × 10− 4 S/cm at 25 °C with the activation energy of 0.3 eV and depends on density. Due to grain boundary resistance dc-conductivity of pellets was lower than 10− 8 S/cm at 25 °C with the activation energy of 0.8 eV. Possible reasons of high grain boundary resistance are discussed.