In this study, a proton conducting phosphate glass, which was fabricated by alkali-proton substitution, was clearly demonstrated to transport protons over a wide oxygen partial pressure in air and hydrogen atmospheres, and in both dry and wet conditions. The proton transport number was confirmed to be unity, regardless of whether the glass was exposed to air or hydrogen atmospheres, in both hydrogen concentration and water vapor concentration cells. The dry hydrogen can be formed by electrochemical hydrogen pumping without current leakage as a result of the presence of other charge carriers. These properties derive from a proton incorporation mechanism, which is non-dependent on defect equilibria, unlike acceptor-doped perovskite-type proton conducting oxides. The advantages of applying this proton conducting glass electrolyte to fuel cells and steam electrolysis is also discussed. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.