A comparative analysis of the isothermal kinetics of water evaporation and equilibrium swollen poly(aciylic-g-gelatin) hydrogel (PAAG) dehydration was performed. The isothermal thermogravimetric (TG) curves of water evaporation and PAAG hydrogel dehydration were obtained in temperature range from 313 K to 353K, at the same conditions. We found that the kinetics of water evaporation could be mathematically described by the model of zero order chemical reaction. The values of the kinetic parameters: E-e,E-a = 29 kJ/mol and ln(A(e)/min = 12.1) for water evaporation were calculated. By the application of the isoconversional method it was found that water evaporation was a kinetically complex process. It was shown that the kinetics of hydrogel dehydration could be described by a linear combination of logistic functions. A novel model, called the nucleation model, able to describe the kinetics of hydrogel dehydration and water evaporation was presented. The rate constants values for nucleation and the multiplication of the nuclei centers (bubbles) at different temperatures were calculated. The values of activation energy of the multiplication of the nuclei centers were calculated. The difference found between the kinetics of water evaporation and hydrogel dehydration was explained by different values for the constant rates of nucleation and multiplication of the nuclei centers during the water evaporation and hydrogel dehydration. (C) 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.