The phase behavior of polystyrene (PS)/poly(vinyl methyl ether) (PVME) blends of various compositions in the presence of compressed carbon dioxide was investigated using in-situ high-pressure fluorescence spectroscopy. Sorption of a few weight percent of CO2, which is a poor solvent for both components but selective for PVME, induces phase separation at 40 and 60 degrees C for all compositions studied. These transitions occur as much as 90 degrees C below the coexistence temperatures for the corresponding binary blends at ambient pressure. The Sanchez-Lacombe equation of state qualitatively predicts phase separation in the PS/PVME/CO2 system as CO2 pressure is increased isothermally. A semiquantitative stability analysis suggests that a disparity in the free volumes of the polymer components upon dilation with carbon dioxide, rather than enthalpic contributions, is the principal driver for phase separation in this system.