The osmotic and activity coefficients of quaternary ammonium salt types are correlated and predicted applying a modified nonelectrolyte UNIQUAC-NRF equation. The excess Gibbs energy of the model consists of the long-range term according to the Pitzer–Debye–Hückel equation and a short-range term expressed through the modified nonelectrolyte UNIQUAC-NRF model. The main difference between the equations of this model and the equations of its counterparts is that through these new equations, the ion or ion pair free electrolyte solutions model can be reduced to the basic nonelectrolyte UNIQUAC model, thus, useable in both nonelectrolyte and electrolyte solutions. The mean activity coefficients of more than 110 binary aqueous electrolyte systems (quaternary ammonium salts and strong aqueous electrolyte solutions) at 298.15 K at 1 bar is calculated through this new model, with two adjustable parameters per electrolyte. These adjustable parameters are applied in predicting osmotic coefficients for the quaternary ammonium salt types. The findings here are compared with that of the Pitzer, modified Wilson, E _NRTL, electrolyte Wilson, UNIQUAC-NRF, Wilson-NRF, NRTL-NRF and Extended UNIQUAC thermodynamic models where its outperformance in all aspects is revealed. It is notable that, by applying the adjustable parameters, the mean activity coefficient for methanol-water-salt systems is correlated at 298.15 K

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