The swelling of a 50 nm monodisperse polystyrene latex by carbon dioxide was measured by dynamic light scattering at 25 degrees C at pressures up to 35 MPa. At the higher pressures, the polystyrene (PS) latex swelled by up to 1.6 times as much as bulk PS. This enhanced swelling is due largely to the adsorption of CO2 into the interfacial region, which lowers the overall interfacial tension between the latex and water phases. On the basis of this enhanced swelling, the thickness of the interfacial region containing PS, carbon dioxide, and water is approximately 4 nm, which is on the order of the radius of gyration of a PS chain.