Miscibility conditions and volumetric properties of solutions of poly(dimethylsiloxane) in supercritical carbon dioxide have been determined as a function of polymer molecular weight, polymer concentration, temperature, and pressure. Measurements have been conducted in a variable volume view cell equipped with an LVDT sensor to identify the position of a movable piston and thus the internal volume of the cell and consequently the density of the solution at a given pressure and temperature. The demixing data (in the form of P-T curies for a given concentration, or as P-x diagrams at a given T) and the density isotherms are presented for solutions of two polymer samples with different molecular weights (M-w = 38,600; M-w/M-n = 2.84 and M-w = 94,300; M-w/M-n = 3.01) at several concentrations in the range from 0.05 to 10 mass % over a temperature range from 302-425 K. Solution densities corresponding to the demixing points also have been identified. Representation of the demixing densities on the density isotherms, i.e., pressure-density plots is a new methodology that gives a direct assessment of the volumetric expansion the solution must undergo before phase separation. The temperature-composition diagrams generated at selected pressures show that the poly(dimethylsiloxane) + CO2 solutions display both lower critical solution and upper critical solution type behavior. The lower critical solution temperature moves to lower temperatures and the upper critical solution temperature moves to higher temperatures with decreasing pressure and they eventually merge together at lower pressures forming an hourglass-shaped region of immiscibility. This behavior is linked to the solvent quality of supercritical carbon dioxide that changes with pressure. (C) 2000 John Wiley & Sons, Inc.