Carboxylic acids in an aqueous effluent of agro-industrial manufacturing can be removed and valorized. Liquid-liquid extraction can be used to choose a selective and efficient solvent. To demonstrate the feasibility of such process, it is necessary to understand the phenomena that are involved. Numerous authors have published predictive physicochemical models of the liquid-liquid equilibrium, based on mass action law and chemical equilibriums susceptible to represent the behavior of the acids in each phase. In this work, the liquid-liquid extraction of four monocarboxylic acids from an aqueous synthetic solution at high concentration ranges has been investigated. Tri-n-butylphosphate diluted in dodecane was the selected system for this application. A physicochemical model that describes the equilibrium data, based on the reaction in the aqueous phase, has been developed, and all equilibrium constants were defined and calculated.