Polycrystalline ceramic materials based on strontium- and magnesium-doped lanthanum gallate exhibit high ionic conductivity compared to the conventional yttria-stabilized zirconia, and find potential application as solid electrolyte for solid oxide fuel cells operating at intermediate temperatures (500-700°C). Sintering of this solid electrolyte is usually carried out at high temperatures being responsible for loss of Ga and consequent formation of impurity phases. In this work, the composition La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ with small additions of strontium gallate was prepared by solid state reaction and the influence of the additive on electrical conductivity and phase composition of the solid electrolyte was investigated. The sintered density of composites is higher than 95% of the theoretical value and the average grain size amounts 2.40 µm after sintering at 1350°C. The contents of impurity phases decreased with increasing strontium gallate additions. The overall ionic conductivity is higher for composites than that of the base material.