A molecular thermodynamic approach is described for predicting polymer glass transition temperatures as a function of the amount of gas sorbed by the polymer. The predictive model is based on a lattice theory of polymer solutions and the concept of order parameters, the use of which has been important in the development of macroscopic models and phenomenological analyses of glass transitions. A general definition of the solvent-induced glass transition is given, and then applied within the framework of this lattice model and its order parameters to predict glass transition temperatures for several polymer-compressed CO2 mixtures. The model is also used to examine a new experimental observation described as retrograde vitrification.