We investigated phase separation by adding different concentrations of MTBE, to the mixtures of naphtha, ethanol and water. The phase separation temperatures of the Naphtha-Ethanol-Water solutions have dropped when the concentration of MTBE increases more. When adding IPA and IBA to the solutions of Gasoline-Ethanol and Gasoline base-Ethanol individually, IBA shows lower temperatures of phase separation than IPA, and it shows synergistic effect when mixtures of IPA and IBA is applied.