Poly(tetrafluoroethylene-co-hexafluoroproplene) (FEP)/polystyrene blends were prepared by the heterogeneous free-radical polymerization of styrene in supercritical (SC) CO2-swollen FEP substrates. Volume incorporations of up to 50% polystyrene were achieved, and the composition and phase morphology of the blends were controlled by varying the styrene monomer concentration and reaction time. The crystallinity and glass transition temperature of the FEP substrate are unaffected by the addition of the polystyrene component, indicating that polymerization occurs exclusively in the amorphous phase and that the polymers are immiscible. The molecular weight of the polystyrene formed within the FEP substrate is significantly higher than that which forms in the SC CO2 phase outside of the substrate. Attempts were made to prepare composite foams by saturation of the blends with SC CO2 and subsequent rapid depressurization. At lower temperatures (conditions under which polystyrene foams) the crystalline domains of FEP prevent expansion. At higher temperatures, in addition to expansion, large-scale phase segregation of the blends occurs.