Ternary mixtures of H2O and CO2 with ethoxylated alcohol (C(i)E(j)) 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 C(8)E(5), C(10)E(6), and C(12)E(6), but not for those mixtures containing either C(4)E(1) or C(8)E(3). Pressure-temperature (PT) projections for this isopycnic three-phase equilibrium were determined for H2O/CO2/C(8)E(5) and H2O/CO2/C(10)E(6) mixtures at temperatures from approximately 25 to 33 degrees 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 H2O/CO2/C(8)E(5) mixtures, the isopycnic pressure at 30.0 degrees 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 H2O/CO2/C(8)E(5) ternary mixtures on the basis of densities measured for the two constituent binary mixtures containing surfactants and from literature values for the H2O/CO2 binary mixture. These predictions show that isopycnic phase formation is a direct consequence of the density maximum calculated for binary mixtures of H2O and CO2 as a function of composition. The enhanced mutual solubility of H2O and CO2 brought about by the addition of surfactant and the pure-component density of the surfactant are also important contributing factors to isopycnic phase formation.