Liquid−liquid phase splitting in ternary mixtures that contain a room-temperature ionic liquid and an alcohol aqueous solutionnamely, [bmim][PF6] + ethanol + water and [bmim][NTf2] + 2-methylpropanol + wateris studied. Experimental cloud-point temperatures were obtained up to pressures of 400 bar, using a He−Ne laser light-scattering technique. Although pressurization favors mutual miscibility in the presence of high concentrations of alcohols, the contrary occurs in water-rich solutions. Both ternary mixtures exhibit a very pronounced water−alcohol co-solvent effect. Solvent isotope effects are also investigated. Phase diagrams are discussed using a phenomenological approach based on a “polymer-like” GE model coupled with the statistical−mechanical theory of isotope effects. The combined effect of a red shift of −15 cm-1 for the O−H deformation mode of ethanol with a blue shift of +35 cm-1 for the O−H stretching mode, both of which occurring after liquid infinite dilution in the ionic liquid, rationalizes the observed isotope effect in the phase diagram. Predicted excess enthalpy (HE) values are inferred from the model parameters. Furthermore, using the Prigogine−Defay equation, an estimation of the excess volumes (VE) is obtained. ; http://dx.doi.org/10.1021/jp034576x