The effects of various surfactants on the interfacial tension between supercritical CO2 and 600 MW polyethylene glycol (PEG) are reported at 45 degrees C on the basis of measurements with a novel tandem variable-volume pendant drop tensiometer. The interfacial tension of the CO2-polyethylene glycol binary system decreases from 9.0 dyn/cm at 85 bar to 3.1 dyn/cm at 300 bar, primarily because of the increase in the density and likewise the free energy density of the CO2 phase. This result is predicted quantitatively with a gradient model using the lattice fluid equation of state. The experimental results for the effects of three surfactants on the interfacial tension are described by a modified Winsor R theory in terms of the molecular interactions on each side of the interface and the surfactant solubility in each phase. At 276 bar, the addition of 1 wt % ammonium carboxylate perfluoropolyether (PFPE) surfactant reduces the interfacial tension from 3.2 to 2.1 dyn/cm and the inter facial area of the surfactant is 437 Angstrom(2)/molecule. In contrast, surface activity is not observed for a hydrocarbon polyether (Brij30), since it favors the organic phase, or for a high molecular weight fluoropolymer (PolyFOA), since it favors the CO2 phase. Because PFPE is interfacially active, it stabilizes PEG-in-CO2 microemulsions.