The primary goal of this work was to identify one-phase regions of the phase diagrams for methanol/H2O/CO2 mixtures and acetonitrile/H2O/CO2 mixtures over the temperature range of 25 degrees-100 degrees C. These mixtures are expected to be useful for efficient separation methods that require polar solutes with fast mass transfer. We were therefore most interested in finding mixture compositions that contained significant quantities of H2O and CO2. For both ternary mixtures at high proportions of CO2, the single-phase region was defined by the onset of an upper critical solution temperature line. Methanol/CO2 mixtures are markedly more miscible with H2O than acetonitrile/CO2 mixtures.