Mean activity coefficient values for NaCl in the mixture (water + ionic liquid) (IL) as solvent (IL being 1-ethyl-3-methyl-imidazolium methanesulfonate, [Emim] [MeSO3] or 1-ethyl-3-methyl-imidazolium ethyl sulfate, [Emim][EtSO4]) were determined from the electrode potential measurements of the cell: Na-(ISE) I NaCl (m(A)), ILs (w), H2O (1 - w) I Cl-(ISE) (ISE means ion selective electrode) at total ionic strengths ranging from 0.10 to 3.20 mol.kg(-1) and at 298.15 K. Different weight fractions (w) of Its (w = 0:01, 0.05, 0.1, 0.2, 0.3 and 0.4) were used. Data were fitted to the Pitzer model and the ion interaction parameters beta(0), beta(1), and C-gamma were calculated. The osmotic coefficients of the solvent (water + IL), the solvent activity, the excess Gibbs free energy for the system were calculated. The negative sign in the excess free energy for the NaCl + IL + H2O solvent mixture at 298.15 K suggests a spontaneous aggregation of {[Emirn][MeSO3] + H2O} or {[Emim][EtSO4] + H2O}.