The interfacial properties of extractant molecules have a significant impact on their complexation reaction activity with rare earth ions at liquid-liquid interface during solvent extraction. Although it is known that acidic organophosphorus extractant exists mainly in the form of dimers in nonpolar organic solvent, the research on solvent extraction kinetics has pointed out that the extractant molecules should react with rare earth ions in the form of monomers at the interface. Therefore, understanding the existing forms of acidic organophosphorus extractant at the interface will help comprehend the interfacial reaction process in solvent extraction. Traditionally, the interfacial properties of the extractant molecules were investigated by measuring interfacial tension isotherms and calculating interfacial adsorption parameters. However, this method can not provide the information of interfacial active species and the aggregation behavior of them. In order to clarify the characteristics of the interfacial behavior of organic extractant molecules at the interface, the effect of subphase pH and the polarity of spreading organic solvent on the adsorption and aggregation behavior of P507 molecules at the air-water interface were investigated by surface pressure-area isotherms and infrared reflectance absorption spectroscopy (IRRAS) based on Langmuir monolayer technique. It was found that P507 monolayers spread by n-hexane at the air-water interface had a certain solubility in the subphase water due to the ionization of the polar groups of P507 molecules. And the solubility decreased as the subphase pH decreased. Thus, the surface pressure-area isotherms changed significantly due to the total amount of P507 molecules remaining on the surface of water changed with the subphase pH. When the subphase pH decreased below 2.0, the influence of the solubility of P507 molecules became inapparent and the amount of P507 molecules remaining on the surface of

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