A single Sanchez-Lacombe lattice fluid equation of state is used to model both phases for a polymer-supercritical fluid-cosolvent system. This method represents well over a wide pressure range both volumetric and phase equilibrium properties for a cross-linked poly(dimethyl siloxane) phase in contact with CO2 modified by a number of cosolvents. A single adjustable binary interaction parameter, obtained from swelling data of the polymer, is used to correlate both polymer dilation and solvent sorption.