The heterogeneous (solid/supercritical fluid solution) free-radical polymerization of styrene in supercritical carbon dioxide-swollen poly(chlorotrifluoroethylene) (PCTFE) polymer film (63 mil) has been studied as an approach to polymer blend preparation. Decompression followed by thermal initiation using AIBN or tert-butyl perbenzoate yields polymer blends with the polystyrene trapped inside the matrix polymer. Polymerization prior to decompression yields more extensively modified products. Polystyrene content and the distribution of polystyrene in the blend can be controlled by adjusting the concentration of styrene in the supercritical fluid or by controlling the time that the PCTFE film is in contact with the fluid. The control imparted is consistent with the phase behavior and absorption kinetics of the styrene-CO2-PCTFE system. Transmission electron microscopy and energy dispersive X-ray analysis (EDX) indicate that the polystyrene exists as discrete phase-segregated regions throughout the thickness of the PCTFE film. Thermal analysis of blends and infrared data from extraction experiments indicate that radical grafting reactions do not occur to any significant extent. EDX data indicate that blends with composition gradients of adjustable degrees of severity can be produced by using soaking periods of shorter duration than is required to achieve equilibrium solubility of the monomer in the substrate.