Chemisorption and physisorption of water from a humid atmosphere, on the oxide Y2O3-doped ZrO2 (YSZ), were investigated by conductivity measurements and thermogravimetry (TG), in the temperature range 35-700 degreesC. Ionic conduction is indicated to take place in a thin layer of water adsorbed on the oxide. That ionic conduction is most probably proton conduction. At T < 600 degreesC, it can exceed the oxygen ion conduction within the YSZ bulk when the oxide is made of fine powder. The electrical conductivity measurements of YSZ fine powder in humid atmosphere are analyzed in terms of water coverage, defect formation and proton mobility in the water layers. Further analysis of the water coverage is done using TG measurements. The enthalpies of defect formation and migration are determined. Results of preliminary conduction measurements of Al2O3 fine powder in a humid atmosphere reveal considerable similarities to those of YSZ.