Measurements of osmotic coefficients of BMimMSO(4) (1-butyl-3-methylimidazolium methylsulfate) and MMimMSO(4) (1,3-dimethylimidazolium methylsulfate) with ethanol, 1-propanol, and 2-propanol at T = 323.15 K are reported in this work. Vapour pressure and activity values for the binary systems studied are obtained from experimental results. The osmotic coefficients are correlated using the extended Pitzer model modified by Archer and the modified NRTL (MNRTL) model. The standard deviations obtained with both models are lower than 0.013 and 0.060, respectively. The parameters obtained with the extended Pitzer model of Archer are used to calculate the mean molal activity coefficients and the excess Gibbs free energy of the binary mixtures. (C) 2009 Elsevier Ltd. All rights reserved.