The hydrophilicity of polymers can be improved through surface modification, traditionally done with conventional organic solvents, but these may cause the deterioration of polymer bulk properties due to irreversible polymer swelling. However, supercritical carbon dioxide (scCO(2)) not only swells the polymer reversibly, but also acts as a convenient transport medium for monomeric solutes, which may be subsequently polymerized. Thus we are able to modify the polymer surface without affecting its bulk strength and durability. Here we report scCO(2) surface modification of polybutadiene using ultraviolet-visible (UV/Vis) and Fourier transform infrared (FTIR) spectroscopy. Monomers such as 3-vinylbenzoic acid and crotonic acid are used to study the effects of pressure, solute structure, and reaction mechanism on the diffusion rates and partitioning of monomers between the fluid phase and the polymer. (C) 2003 Wiley Periodicals, Inc.