An understanding of the monomer reactivity ratios is extremely useful for tailoring copolymers with desired compositions for a diversity of end-use applications. In situ ATR-FTIR was employed to monitor the initial formation of copolymers of ethylene and vinyl acetate (VAc) during polymerization in the "green" solvent supercritical carbon dioxide (scCO(2)). To obtain the copolymer composition, a calibration curve of the absorbance ratio 1737/2929 cm(-1) versus vinyl acetate content of poly(ethylene-co-vinyl acetate) (PEVA) was established, covering the range of 5-98 wt %. The reactivity ratios for copolymerization of ethylene and VAc in scCO(2) were determined using both the Kelen-Tudos and the nonlinear least-squares (NLLS) methods. Off-line H-1 NMR analysis was also utilized for obtaining the copolymer composition, by which the determined reactivity ratios were compared with the ones obtained using in situ FTIR. The effect of reaction temperature and pressure on reactivity ratios was examined at 50 and 72 degrees C and at 13.8 and 27.6 MPa, with the lower temperature and higher pressure conditions increasing the reactivity ratios slightly. In situ FTIR was found to be able to accurately measure the reactivity ratios, and both the Kelen-Tudos and NNLS methods gave similar results. The obtained reactivity ratios will promote the application of the green solvent scCO(2) in the production of the widely commercialized copolymer PEVA.