CO2 + polymer phase behavior is of interest in a number of applications, including the precipitation of polymer particles for drug delivery and the deposition of fluoropolymer films. There is experimental evidence to suggest that CO2 forms weak complexes with carbonyl groups in these polymers, and it has also been suggested that specific interactions between CO2 and the lone pair of electrons on fluorine in a C-F bond contribute to the high solubility of CO, in these fluropolymers. In the present work, therefore, we have quantified the strength of weak complexes in CO2 + poly(lactide-co-glycolide) (PLGA) systems using in situ FTIR spectroscopy; and calculated cloud points in these systems using the measured FTIR spectra combined with a modified lattice model. We demonstrate that this model is able to predict the phase behavior Of CO2 + PLGA copolymer systems using only one parameter obtained from a reference system (CO2 + low molecular weight PLA in our case). Finally, we have extended. the model to the calculation of phase behavior in CO2 + poly(fluroalkyl acrylate) systems, and show that the results may be used to infer the existence of weak CO2-C-F complexes in these systems. (c) 2007 Elsevier B.V. All rights reserved.