In the present study we have demonstrated the unique capability of near-infrared/infrared (NIR/IR) spectroscopy to characterize the hydrogen bonding of the OH groups in ethylene-vinyl alcohol copolymer (EVOH). Fourier transform (FT)-NIR/IR spectroscopic measurements were made of EVOHs with ethylene contents of 33, 49, 75, and 89 mole %. A strong and broad absorption at ~4780 cm<sup>-1</sup>, which is the first combination of the OH stretching and deformation vibrations, is assigned to the aggregated OH groups. The band shows a shoulder at 4970 cm<sup>-1</sup> at an ethylene content of 49 mole %, which becomes a distinct peak at a content of 89%. This feature is assigned to the free OH group isolated in the surroundings of the ethylene component. Existence of the free OH group is additionally evidenced by the appearance of sharp absorptions of the OH stretching fundamental at 3027 cm<sup>-1</sup> and its first overtone at 7094 cm<sup>-1</sup>. The relative population of the free OH has been estimated as 0, 0.3, 1.5, and 3.9 mole % for ethylene content of 33, 49, 75, and 89, mole %, respectively.

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