A new associated lattice-fluid equation of state is derived for CO2 + polymer mixtures by incorporating complex formation in the Guggenheim-Huggins-Miller lattice-fluid partition function. The equation contains mixture parameters that Must be obtained from experimental data but are independent of the temperature and pressure Moreover, one Of the parameters may be Obtained via in situ attenuated total reflection Fourier transform infrared measurements. The solubilities of CO2 in several polymers were correlated over a wide range of temperatures and pressures using only one adjustable parameter. The extent of swelling of the polymers due to the addition of CO2 could then be predicted without any additional parameters.