Samples of LixLa3Zr2O8.5+0.5 (x = 6, 7, 8, 9, 10) were synthesized with the citrate-nitrate method. Neutron diffraction studies have shown the presence of lithium carbonate impurities in the synthesized compounds. We propose a simple and effective method to determine the carbonate impurity content in the solid electrolytes. The technique is based on the measurement of the carbon dioxide volume produced from the interaction of the investigated material with acid. Determined in this way, the content of Li2CO3 in the synthesized electrolytes Li7La3Zr2O12, Li8La3Zr2O12.5 and Li9La3Zr2O13 are 1.32 +/- 0.04, 1.95 +/- 0.06 and 3.49 +/- 0.10 wt.%, respectively. From the obtained data, the actual lithium content per formula unit of complex oxide was calculated for the synthesized compounds. The composition with x = 9 had the highest total conductivity, sigma= 7.5 x 10(-6) S cm(-1), at room temperature. All the investigated electrolytes have an activation energy of approximately 50 kJ mol(-1). (C) 2014 Elsevier Ltd. All rights reserved.