High-frequency dielectric relaxation measurements up to 20 GHz were conducted in aqueous solutions of poly(vinyl alcohol)s (PVAs) to examine their hydration structures and dynamic behavior. Dielectric relaxation spectra of aqueous PVA solutions decomposed well into three sets of the Debye-type relaxation modes. This was attributed to the rotational relaxation mode of free water molecules, an exchange mode of water molecules hydrated to hydroxy (OH) groups for bulk water, and local motions of PVA main chains. OH groups of PVAs keep ca. 2-2.2 hydrated water molecules in aqueous solution, in contrast to the hydration behavior of 1,4-cyclohexanedimethanol and 1,4-cyclohexanediol model molecules possessing OH groups that lack intramolecular hydrogen bond formation, which keep ca. 5 water molecules per OH group in aqueous solution. In the case of PVAs, the formation of intramolecular hydrogen bonding between mainly adjacent OH groups effectively diminishes their hydration numbers.