The modified adsorption isotherm model, incorporating the Stokes-Robinson modification of the Brunauer-Emmett-Teller (BET) adsorption isotherm, is used to calculate the activity of the organic acid and water, and the liquidus curve. This is the first known use of the modified adsorption isotherm for estimation of thermodynamic information for aqueous organic acid solutions and one of few works using the model for predicting information for aqueous acids in general. The method described here represents a simplified approach to estimating thermodynamic properties. The model offers some improvement over regular solution theory, but perhaps more importantly offers a method for adjusting thermodynamic properties in the presence of other components in the future. While other models such as UNIQUAC may prove to be more accurate, this model provides satisfactory results with more ease.