Due to the presence of disordered oxygen sites in the structure of Ba5Y13[SiO4]8O8.5, this material was tested by the means of alternating current (AC) impedance spectroscopy to assess its ionic conductivity. Below the temperature of 873 K, the resistance values are too large to be measured. Data were collected in air, oxygen and argon flow, but only slight improvement of the conductivity was observed for the oxygen environment (see Table ). The Ba5Y13[SiO4]8O8 shows an ionic conductivity of 5.79 × 10 -6 S cm -1 at 873 K which increases up to 4.48 × 10 -4 S cm -1 at 1073 K in oxygen atmosphere. These values are higher than recorded for the apatite-type Y10(SiO4)6O3 (6.8 × 10 - 8 S cm -1 at 973 K). The Arrhenius plots of ionic conductivity are shown on Figure , and the calculated activation energy of Ba5Y13[SiO4]8O8.5 is 1.87 eV for oxygen environment (Table ) which is close to the activation energy reported for Y2Si2O7 (1.04-1.68 eV). For reference, one of the best silicate ion conductors, apatite-type La9.333Si6O26 reaches much higher conductivity values at 673 K (3.99 × 10 -4 and 2.41 × 10 -6 S cm -1 for parallel and perpendicular measurements with respect to a single crystal) and lower activation energy of 0.66-0.773 eV. These values could be improved by engineering Sideficiencies; therefore, similar approach can be used to increase the conductivity of Ba5Y13[SiO4]8O8.5 in the future.