The self-diffusivities of high molecular weight polystyrene chains in CO2-swollen polystyrene matrices were measured in real time using neutron reflectivity. Bilayer films of hydrogenated and deuterated polystyrene (PS) were prepared on silicon substrates and exposed to compressed CO2 The broadening of the interface between the films as a function of time was determined from the reflectivity profiles, yielding the chain diffusivity. Diffusivity was studied as a function of polymer molecular weight, concentration Of CO2 in the polymer film, and temperature. Nearly an order of magnitude enhancement in the diffusivity of polystyrene chains (M = 2 x 10(5)), from 1.62 x 10(-16) to 9.35 x 10(-1)6 cm(2)/S, was found with a modest increase in the concentration of CO2 in the polystyrene (from 8.9 to 11.3 wt %) at 62 degreesC. This concentration dependence was modeled using the Vrentas-Duda free volume theory. 50-52 At a constant temperature and CO2 pressure the polystyrene diffusivity scaled as M-2.38. The scaling of the self-diffusivity of PS in CO2-swollen PS with T - T-g, where T-g is the glass transition temperature depressed by the presence of the solvent, is discussed.