The nano-grained specimens of yttria-doped zirconia have been fabricated via a combination of low-temperature nano-powder synthesis and room-temperature high-pressure (4 GPa) compaction. The microstructure is essentially free from macroscopic pores but involves interfacial hydrated layers, which facilitate adsorption of water molecules within the specimens. The three kinds of proton-containing species, i.e., surface terminating OH groups, H-bonded H2O molecules and free H2O molecules, have been distinguished from each other by the Thermal Desorption Spectroscopy analysis in terms of thermal stability, and also by the H-1 MAS-NMR measurements. The electric conduction in humidified atmospheres is dominated essentially by proton hopping below ca. 800 K, which is verified by the H/D isotope effect and water vapor pressure dependence. The effect of grain growth on the conductivity suggests that the protonic conduction path is the grain boundary or interface. The present study shows a good feasibility of fabricating proton-conducting materials based on nano-grained oxides, even if the bulk property involves negligible proton solubility and conductivity, by the formation of grain boundary network of interfacial hydration layer. (C) 2011 Elsevier B.V. All rights reserved.