The effect of H2O molecules on the electrical conductivity of vanadium pentoxide hydrate, V2O5 . nH(2)O, was investigated in connection with their orientation through polarized FT-IR spectroscopy. It was confirmed that interlayer H2O molecules are isotropic in the ab plane of the layered structure, indicating that they are adsorbed randomly between the layered structures. This is in contrast to the anisotropy of electrical conductivity, which is related to the difference in the hopping distances between the a and b directions. The structure of V2O5 . nH(2)O in a humid condition was also studied by means of FT-IR, ESR, and XRD measurements, which should strongly affect the electric conductivity. The infrared polychroic property of the skeletal vibrations of the sample remained unchanged with varying H2O content. However, the ESR spectra of V4+, a trace amount of which is intrinsically present in the sample, indicated that a free rotational motion of V4+ occurs when a double layer of H2O was formed. This substantiates the idea that some of the V4+ ions, in additino to the V5+ ions, are present in the interlayer spaces rather than entirely in the skeletons.