This paper describes the interaction of water with solid polymers. A portion of the water adsorbed in hydrophilic materials is strongly bound to individual sites (bound water), while an additional amount is bound less firmly to the polymeric chain (free water). Both, free and bound water, have been the subject of many studies that have uncovered, in some cases, qualitative evidence of the presence of bound water. In this paper we report a method to quantify the fraction of water adsorbed in the first layer in contact with an hydrophilic material. The measurement was carried out on infrared spectroscopy data of methylcellulose films exposed to several humid environments. The quantification was based on the determination of the change in the natural vibration frequency brought about by the interaction of water molecules with the solid. (C) 2003 Elsevier Science Ltd. All rights reserved.