Certain alkyl ammonium salts are called ionic liquids They can be used as green solvents without toxicity, effects on the ozone layer depletion, effects on climate change, and any other significant environmental impacts; in this regard knowledge about their thermo-physical properties are really important. In this work activity coefficients of aqueous solutions of alkyl ammonium salts at 298.15 and 293.15 K are modelled using the Extended UNIQUAC (Universal Quasi Chemical) thermodynamic model. Adjustable parameters of the Extended UNIQUAC thermodynamic model were obtained by non-linear regression between experimental data and the results calculated from the model. The results of the model are in good agreement with the experimental values in most cases, the average absolute deviation of the model being less than 2 , which is acceptable considering the uncertainty of the experimental data. Additionally, the results gained from the Extended UNIQUAC model are also compared with the results obtained from Electrolyte NRTL and Pitzer thermodynamic methods. © 2016, Springer Science+Business Media New York.