Osmotic coefficients phi, mean activity coefficients gamma(+/-), vapor pressure p data, and excess Gibbs free energies G(E) of aqueous solutions of three ionic liquids 1-propyl-3-methylimidazolium bromide [PMIm]Br, 1-pentyl-3-methylimidazolium bromide [PnMIm]Br, and 1-hexyl-3-methylimidazolium bromide [HMIm]Br were determined by the vapor pressure osmometry method at four temperatures (298.15K to 328.15 K) in intervals 10 K. From the comparison of osmotic coefficients it follows that aqueous solution of [PMIm]Br shows a more pronounced deviation from Debye-Huckel limiting law (DHLL) and vapor pressure depression more than the other studied ionic liquids which was interpreted in terms of hydrophobic interactions. The Pitzer-ion interaction and MNRTL electrolyte models satisfactorily correlate experimental osmotic coefficient data with good precision. The parameters of the Pitzer-ion interaction model are used to calculate the mean molal activity coefficients and excess Gibbs free energies. (C) 2008 Elsevier Ltd. All rights reserved.