We present a compressible lattice model for representation of both low-pressure and high-pressure phase equilibria in CO2 + polymer systems. The model explicitly accounts for weak complex formation in these systems, using two parameters that are obtained by fitting experimental cloud point pressures or sorption equilibria. In addition, one of these parameters may be obtained from Fourier transform infrared (FTIR) spectroscopy measurements. We demonstrate the application of this model to the prediction of low-pressure sorption behavior in the CO2 + poly(vinyl acetate) (PVAc) system, using parameters obtained from high-pressure cloud point data. In addition, we show that we are able to predict the sorption behavior of CO2 + PLGA copolymer systems using a single parameter obtained from CO2 + poly(lactic acid) (PLA) cloud point data, together with enthalpies of association obtained from FTIR measurements.