We have determined the activity coefficients for NaCl in NaCl + NaForO + NaAcO + H2O and NaCl + NaForO + NaPrO + H2O mixtures, and for NaBr in the NaBr + NaForO + NaAcO + H2O mixture (ForO = Formate, AcO = Acetate and PrO = Propionate). The studies were performed by the emf method and at 25 degrees C. The analysis of the experimental data was accomplished through a Reilly-Wood-Robinson type treatment, by replacing the Friedman parameters by the I-dependence functions defined by the Scatchard, Fitter and Lim models. The methodology used consisted of introducing the binary interaction parameters, obtained from previous studies of the corresponding ternary mixtures NaX + NaY + H2O (X : Cl or Br; Y : ForO, AcO or PrO), into the equations for the quaternary system. In the present paper, the binary interaction parameters corresponding to the mixture NaCl + NaForO + H2O have been more accurately re-determined. The data thus obtained from the use of the different models were compared to each other. The mixing parameters for the NaForO + NaAcO + H2O and NaForO + NaPrO + H2O ternary systems were also determined as ionic strength functions.