Isobaric vapor-liquid equilibrium (VLE) data were measured for the ternary system {ethanol (1) + water (2) + 1,3-dimethylimidazolium dimethylphosphate (3) ([MMIM]+DMP-)} at atmospheric pressure (101.3 kPa). The measurement was performed with a modified Othermer still. The results indicated that the VLE of the {ethanol (1) + water (2) + [MMIM]+DMP- (3)} ternary system was clearly different from that of the {ethanol (1) + water (2)} binary system. The IL studied showed a slight crossover salt effect, which led to a change of the relative volatility of ethanol to water and even to the elimination of the azeotropic phenomenon of ethanol and water. The measured ternary VLE data were correlated using NRTL.