The CO2 sorption and the consequent swelling of un-cross-linked poly (dimethylsiloxane) (PDMS) have been measured simultaneously at high pressure using in-situ ATR (Attenuated Total Reflectance)-IR spectroscopy. The absorbance of the upsilon(3) (2335 cm(-1)) band Of CO2 dissolved in PDMS was used to determine quantitative sorption data. The absorbance of the IR band of PDMS was used to calculate the polymer swelling. Data at 25 degreesC and 50 degreesC for pressures up to 12 MPa are reported and compared to literature values. A shift of the delta(CH3) band of PDMS (at 1259 cm(-1)) was observed with the increase of sorbed CO2 and was related to the swelling of the polymer. This is a novel use for high-pressure in-situ ATR-IR spectroscopy, which allows the simultaneous measurement of gas sorption and polymer swelling and provides molecular-level insight into the behavior of polymers under high-pressure or supercritical CO2.