Phase behavior and small-angle neutron scattering (SANS) experiments were performed on mixtures of D2O/CO2/n-hexaethylene glycol monododecyl ether (C(12)E(6)) as a function of temperature (25-31 degrees C), pressure (63.1-90.7 bar), and CO2 composition (0-3.9 wt %). The addition of carbon dioxide shifts the liquid-liquid miscibility gap present in C(12)E(6)D(2)O mixtures to lower temperatures. Parameters extracted from model fits of the SANS spectra. indicate that, while micellar structure remains essentially unchanged, critical concentration fluctuations increase as the phase boundary and plait point are-approached. The variations of micelle structure, scattering contrast, and particle volume fraction with pressure, temperature, and carbon dioxide concentration are consistent with only a minimal amount of carbon dioxide entering the micelles under all conditions.