Raman spectra of poly methyl methacrylate (PMMA) in contact with compressed CO2 were measured, to investigate effects of CO2 sorption on structure of the PMMA chain. The solubility and diffusivity of CO2 in PMMA were estimated from temporal variations of intensities of CO2 peaks. The PMMA structure was analyzed from temporal variations of vibrational energies of PMMA peaks. The results show that the vibration energies of C-H stretching modes of PMMA increase with CO2 sorption, whereas those for skeletal vibration modes decrease. These energy shifts are attributed to elongational deformation of PMMA. The PMMA structure is deformed with stretching of the chains as a bundle. From energy shifts of the CO2 peaks, the size of the CO2 accommodated space between the bundles is estimated to be 1.6-1.9 nm. Furthermore, it was observed that the vibrational energies of the PMMA modes in foamed glassy PMMA differ from the values in glassy PMMA without foams. This result suggests that the local structure of the PMMA chain changes with the process of the CO2 sorption and/or foaming. The local structure of the PMMA chain might be one of the dominant factors governing the properties of cellular materials. (C) 2008 Wiley Periodicals, Inc.