Conductivity of a Mg2+-ion-conducting solid oxide Mg0.5-x(Zr1-xNbx)(2)(PO4)(3) (x = 0.15) was measured with non-blocking Mg metal electrodes at 350 degrees C in vacuum. The oxidation of Mg electrodes was suppressed by preventing their exposure to air throughout the deposition and measurements. The observed conductivity agreed with conductivities measured by ac impedance spectroscopy with blocking Pt electrodes. The transport number of Mg2+ ions derived from the conductivities measured with Mg electrodes by dc polarization method and ac impedance spectroscopy was similar to 1. These observations evidenced that this compound is a pure Mg2+-ion conductor and stable in contact with Mg. (C) 2017 The Electrochemical Society. All rights reserved.