Unrestricted Primitive Model (UPM) has been applied to predict various properties of room-temperature ionic liquids (RTILs) systems. Five RTILs with different characteristics are selected for this study: 1-etyl-3-methylimidazolium ([Emim][X], X = iodide, chloride, tri-fluoromethanesulfonate, trifluoroacetate, and ethyl sulfate) aqueous solutions. Monte Carlo (MC) simulations are carried out in the NVT ensemble to calculate the thermodynamic properties. Binary mixtures i.e. [Emim][Cl] + water are studied in three temperatures (298.15 K. 308.15 K and 318.15 K) to calculate internal energy and osmotic coefficient. Moreover, activity of solvent, osmotic coefficient and activity coefficient for [Emim][Otf] + water and [Emim][Tfa] + water are reported at 298.15K. Available experimental values of osmotic coefficients for [Emim][EtSO4] are compared with simulation results. In agreement with previous experimental studies, our results support the validity of the UPM to investigate aqueous solutions of RTILs. (C) 2012 Elsevier B.V. All rights reserved.