Vapor pressures, heat capacities and densities of the 1-ethyl-3-methylimidazolium ethyl sulfate (EMISE) + water were measured at the various temperatures and ionic liquid concentrations. The system was chosen as a novel candidate for organic working pairs for an absorption heat pump. The vapor pressures of the solution were correlated by the NRTL model, the heat capacity and density of the solution were correlated by the polynomial equation as a function of temperature and concentration, and the parameters in the regression equation were determined by a Levenberg-Marquardt method, the average relative deviations between the experimental and calculated values in vapor pressure, heat capacity and density measurements were 1.9%, 1.2%, 0.8%, respectively. The vapor pressures of the solution decreases with the decrease in solution temperature and with the increase in EMISE mole concentration, the heat capacity of the solution decreases with EMISE mole concentration at the same temperature and slightly increases with increasing temperature at the same concentration. The density of the solution decreases with increasing temperature and water mole concentration. (C) 2009 Elsevier B.V. All rights reserved.