Supercritical fluid (SCF) carbon dioxide processing has been studied as a new route to polymer surface modification. Three fluoropolymer substrates were studied : poly(tetrafluoroethylene) (PTFE), poly(chlorotrifluoroethylene) (PCTFE) and poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) using a two-step modification procedure. In the first step, blends or semiinterpenetrating networks were prepared by carrying out a free radical polymerization of styrene (or styrene with divinylbenzene or triallyl cyanurate) within the SCF-swollen polymer substrate. The second step involved sulfonating the polystyrene using heterogeneous (solution-solid) sulfonation conditions. Composition of the surfaces was determined by X-ray photoelectron spectroscopy, and wettability was assessed using water contact angle measurements. Depressed advancing and receding contact angles indicate increased wettability of all of the fluoropolymer samples.