Dielectric relaxation measurements have been performed on 75 μm thick polyethylene terephthalate (PET) films. At 1 kHz frequency, samples treated by 55% and 100% relative humidity (RH) showed loss tangent increase of 28% and of 56%, respectively. Confirmed by the dielectric constant results, the increased strengths of the relaxation were linearly related to the percentage RH within experimental error. The activation energy of molecular polarization at 100% RH was determined by the Arrhenius relation to be 97.9 kJ/mol. This value is 36% greater than that of the dry and 55% RH samples, indicating that the water molecules are restricted between the polymeric chains. Possible mechanisms including van der Waals bonding of the water molecules to the terminal ends of the polymer chains or to carbonyl groups are suggested.