Electrical properties of vanadium pentoxide hydrate were studied by impedance measurements. Of the intrinsic samples prepared by the ion-exchange polymerization method, an ionic conduction predominates over electronic conduction at around the relative H 2 O vapor pressure of 0.1, i.e., at the monolayer adsorption of H 2 O molecules, and increases to 80% of the total conductivity at higher H 2 O vapor pressures. Impedance analysis of the system indicated that H 2 O molecules are adsorbed in two modes at a humidity of P/P 0 <0.4, one of which restricts the motion of H 2 O molecules and eases with increased humidity up to P/P 0 =0.4 to have H 2 O molecules relax similarly to the other mode having a smaller relaxation time for H + hopping