The interaction between water vapor and fluorite-type oxide ion conductors, and its effects on surface and electrochemical properties are investigated. High deuterium ion intensity was observed for rare-earth doped ceria annealed in D2O-containing atmospheres. It indicates that a pretty amount of deuterium is absorbed in the ceria polycrystalline. The oxygen isotope exchange rate on YSZ surface increased with water vapor pressure, indicating that the interaction between water and oxygen vacancy in YSZ is significant. The interfacial conductivity at platinum/YSZ interface in air at T=968 K was drastically improved by addition of 1 kPa of water vapor pressure. This improvement was clearly observed for YSZ electrolyte, however, only a small effect was observed for Ce0.8Gd0.2O1.9. (C) 2004 Elsevier B.V. All rights reserved.