The water sorption isotherms of two gellan gum samples, i.e. a low acyl gellan gum and a high acyl gellan gum, were determined using an isopiestic method in the range of water activity between 0.1 and 0.98 and at a temperature of 25 degrees C. The experimental sorption data were fitted to several models, including the BET, GAB, Caurie, Harkins and Jura, Smith, Chung and Pfost, Khun and Oswin models. The Caurie model and the Oswin model were found to best represent the experimental data through the entire range of water activity. In the range of water activity less than 0.75 the Chung and Pfost equation was shown to give the closest fit to the experimental data. The content of water adsorbed in the monomolecular layer was calculated and the area of I g adsorbent that contains water molecules in the monomolecular layer was evaluated. The constant related to the heat of sorption for the first and subsequent layers of molecules, and the constant related to the heat of condensation of pure water was estimated. The partial Gibbs free energy per kilogram of adsorbent was evaluated and found to be significantly different for the two samples of gellan gum. (c) 2005 Elsevier Ltd. All rights reserved.