Vapour pressure data were measured for three binary systems containing water, methanol or ethanol with an ionic liquid (IL) 1-ethyl-3-ethylimidazolium diethylphosphate([EEIMI[DEP]) and for three ternary systems, i.e. (water + ethanol + [EEIM][DEP]), (water + methanol + [EEIM][DEP]), and (ethanol + methanol + [EEIM][DEP]), at varying temperature and IL-content ranging from mass fraction of 0.10 to 0.85 by a quasi-static method. The vapour pressure data of the binary systems were correlated by NRTL equation with average absolute relative deviation (ARD) within 0.0091. The binary NRTL parameters were used to predict the vapour pressure of the ternary systems (ethanol + water + [EEIM][DEP]), (water + methanol + [EEIM][DEP]), and (ethanol + methanol + [EEIM][DEP]) with an overall ARD of 0.037 and the maximum deviation of -0.1295. The results indicate that ionic liquid [EEIM][DEP] can give rise to a negative deviation from the Raoult's law for the solvents of water, methanol and ethanol, but to a varying degree leading to the variation of relative volatility of a solvent and even removal of azeotrope for (water + ethanol). (c) 2006 Elsevier Ltd. All rights reserved.