Vapor pressure data were measured for nine binary systems containing water, methanol or ethanol with the ionic liquids 1-methyl-3-methylimidazolium dimethylphosphate ([MMIM][DMP]), 1-ethyl-3-methylimidazolium diethylphosphate ([EMIM][DEP]) and 1-butyl-3methylimidazolium dibutylphosphate ([BMIM][DBP]) and one ternary system ethanol-water-[MMINl][DMP] at varying temperature and ionic liquid mass percent ranging from 10% to 70% by a quasi-static method. The vapor pressure data of the binary systems were correlated by NRTL model with overall average absolute relative deviation (ARD) of 0.89% and a maximum ARD within 2%. Based on the vapor pressure depression of binary systems interpolated at ionic liquid mole fraction 0.05 in the temperature range from 280 to 370K, it is shown that the effect of ionic liquid on the vapor pressure lowering follows the order [MMIM][DMPI > [EMIM] [DEP] > [BMIM][DBP] for water, and [BMIM][DBP] > [EMIM][DEP] > [MMIM][DMP] for methanol and ethanol, suggesting that the "ionic" characteristic of the ionic liquid dominates in water, while the "molecule" characteristic prevails in organic solvent. The binary NRTL parameters were used to predict the vapor pressure of the ternary system ethanol-water-[MMIM][DMP] with an ARD of 2.8% and the maximum relative deviation of -5.43%. (c) 2006 Elsevier B.V. All rights reserved.