An oxygenated hydrocarbon-based polymer, poly(propylene glycol) monobutyl ether (PPGMBE), is very CO2-Soluble. Therefore PPGMBE may serve as the CO2-philic portion of CO2-soluble surfactants, dispersants, thickeners, and chelating agents. CO2-PPGMBE isothermal pressure-composition phase diagrams were determined at 298 K for PPGMBE average molecular weights of 340, 1000, and 1200. All the diagrams share the same features of one vapor-liquid-liquid three-phase equilibrium line and three two-phase equilibrium regions. The mixture critical points for the pseudobinary solutions were determined to be (7.2, 31.0, and 41.7) MPa for molecular weights of 340, 1000, and 1200, respectively.