The aprotic Na-O-2 battery has attracted significant interest over the past few years because of its high theoretical specific energy and uncompromised round-trip efficiency. So far, most of the Na-O-2 studies, using a cell configuration of Na metal anode, aprotic Na+ electrolyte and porous O-2 cathode, focus on the O-2 reactions in the cathode, which, however, could be complicated by the use of Na metal anode. This is because both the electrolyte and O-2 (from cathode) can react with the Na metal anode and some of the side reaction products could cross over to the cathode and interfere with the O2 reactions occurring therein. Toward a better understanding of the O-2 electrochemistry in aprotic Na-O-2 batteries, it is essential to set up a clean electrochemical system, i.e., using counter/reference electrodes that react with neither electrolyte nor O-2. In this report, Na3V2(PO4)(3)/NaV2(PO4)(3) couple having a formal potential of 3.35 V vs. Na/Na+, reacts with neither electrolyte nor O-2 during its multiple cycles of intercalation/deintercalation process, and therefore has been identified to be a stable counter/reference electrode for the study of O-2 electrochemistry in aprotic Na-O-2 batteries. (C) 2016 The Electrochemical Society. All rights reserved.