Ternary mixtures of HiO and CO2 with ethoxylated alcohol (C,-E/) surfactants can form three coexisting liquid phases at conditions where two of the phases have the same density (isopycnic phases). Isopycnic phase behavior has been observed for mixtures containing the surfactants CsEj, CioEo, and CnEe, but not for those mixtures containing either C4E| or CsEa. Pressure-temperature (PT) projections for this isopycnic threephase equilibrium were determined for FhO/CCVCgEs and IO/CCVCioEo mixtures at temperatures from approximately 25 to 33 °C and pressures between 90 and 350 bar. Conditions for density-matched phases were measured by observing an inversion of the two liquid phases of interest as a function of pressure at constant temperature. For the HiO/CCVCgEs mixtures, the isopycnic pressure at 30.0 °C was also determined by measuring densities of these two phases with increasing pressure across the inversion. This pressure was found to be in very good agreement with the value obtained by observing the phase inversion. A recent modification of the Peng-Robinson equation of state was used to predict liquid densities for FO/CCVCgEs ternary mixtures on the basis of densities measured for the two constituent binary mixtures containing surfactants and from literature values for the FO/CO binary mixture. These predictions show that isopycnic phase formation is a direct consequence of the density maximum calculated for binary mixtures of H2Ö and COi as a function of composition. The enhanced mutual solubility of H2U and COj brought about by the addition of surfactant and the pure-component density of the surfactant are also important contributing factors to isopycnic phase formation.