The description of permeability and solubility of CO2 in poly(ethylene Lerephthalate) both in the glassy and in the rubbery state is analyzed by means of thermodynamic based models for solubility and diffusivity. The Non-Equilibrium Thermodynamic approach for Glassy Polymers (NET-GP), together with the Sanchez Lacombe equation of state, provides the description of the solubility isotherms above and below the glass transition temperature (NELF model). The penetrant diffusion coefficient is then taken as the product of a kinetic factor, the mobility, and a thermodynamic factor associated to the concentration dependence of the chemical potential of the diffusing species. The thermodynamic factor is predicted by the NELF model, while the mobility factor is considered to depend exponentially from penetrant concentration. following the usual trend commonly found experimentally, and its expression contains only two adjustable parameters. The model analysis shows that, in all the cases examined, the model used is able to describe carefully the experimental trends in a simple and effective way, accounting for all the different behaviors observed, for both solubility and permeability. (C) 2013 Elsevier B.V. All rights reserved