A drastic structure change during a hydration process of a poly(methacrylic acid) (PMAA) film was observed by time-resolved in situ attenuated total reflection infrared (ATR-IR) spectroscopy. Inter-or intra-hydrogen-bonds via side-chain carboxyl groups are formed as cyclic dimers, side-on dimers and linear open chain oligomers including open dimers in a dried PMAA film. By contacting water, the side-chain hydrogen-bonds in PMAA are dissociated instantly, and then the subsequent carboxyl groups which have no hydrogen-bond newly hydrate with water molecules in a side-on form. Quantum chemical calculations (QCCs) using a model monomer of propionic acid (PA) also support the hydrogen-bonded and hydrated structures explored by the ATR-IR spectroscopy. It has been concluded from the present study that the dissociation of hydrogen-bonded and newly created hydrated structures via the carboxyl groups play an important role for the swelling of PMAA in water. (C) 2009 Elsevier Ltd. All rights reserved.