A thermodynamic model is proposed to describe the sorption of water in Nation based on the Flory-Huggins activity model and an appropriate osmotic pressure correction term for the chemical potential of water within the swollen membrane. The key variables for sorption are equivalent weight of ionomer, acid strength of the ionic groups, modulus of polymer elasticity, and interaction between water and polymer. The water uptake per unit mass of dry Nation increases with the increasing acid strength of the functional groups, decreasing Young's modulus, and decreasing equivalent weight of Nation. The model provides insights into the sorption and swelling behavior of ion-exchange membranes, and thus, may be useful in evaluating and designing alternate proton-exchange membranes for fuel cell applications. In a companion paper (Part II), a predictive model is presented for the transport of protons in Nation. (c) 2005 The Electrochemical Society. All rights reserved.