Interfacial tensions between water/n-hexane and water/toluene in the synthesis of ether-ester compounds by phase-transfer catalysis were measured. These two-phase systems contained phase-transfer catalysts, an aqueous-phase reactant, and/or alkaline. It was shown that the measured data could be well described by the Gibbs adsorption equation coupled with the Langmuir monolayer isotherm. In addition, the interfacial parameters such as the surface excess and adsorption constant determined in one-compound systems allowed one to predict the surface pressure of the present practical liquid-liquid phase-transfer-catalyzed systems. Other interfacial properties of species such as the molecular area and ion-pair dissociation constant were also evaluated.