The most important effect of sorption of compressed gases and supercritical fluids into glassy polymers is the reduction of the glass transition temperature (T-g). This plasticization effect causes changes in mechanical and thermophysical properties of the polymers. In this work, a thermodynamic study based on experimental and theoretical results is addressed. New data were carried out for poly(2,6-dimethylphenylene oxide) (PPO), poly(acrylic acid) (PAA), and the copolymer vinylpyrrolidone-vinyl acetate [P(VP-VA)] using an inverse gas chromatographic technique. To model the T-g behavior of diluent-polymer systems, a model that couples the lattice-fluid equation of state and the Gibbs-DiMarzio criterion was used. A parametric study of the influence of the physical properties (lattice coordination number, molecular weight, and binary interaction parameter) on the T-g behavior is presented. The thermodynamic model fairly describes the experimental data measured in this work and gives a phenomenological representation of the retrograde vitrification for the systems PPO-CO2, PVP-CO2, and P(VP-VA)-CO2.