Equilibrium constants measured from the nu(2) bending mode of CO2 by FTIR spectroscopy are reported for the electron donor-acceptor interactions of CO2 with three Lewis bases : triethylamine(TEA), pyridine (PYR), and tributyl phosphate (TBP). The average K-c values are 0.046 (CO2-TEA), 0.133 (CO2-PYR), and 1.29 (CO2-TBP) L/mol at 25 degrees C in the solvent pentane. For the CO2-TBP system, the average enthalpy of association, Delta H degrees, is -4.7 kcal/mol. Ab initio calculations indicate that steric repulsion of the ethyl groups in TEA cause the binding energy of the CO2-TEA complex to be weaker than that of the CO2-PYR complex by 1.34 kcal/mol, a trend that is in agreement with our spectroscopic data. The lattice fluid hydrogen bonding model was used in conjunction with the spectroscopically determined K-c values to predict bubble points for the CO2-TEA and CO2-TBP systems and CO2 sorption in a hypothetical polymer. These calculations indicate that these relatively weak specific interactions have a measurable effect on phase behavior and can influence sorption of CO2 in polymers.