Interfacial tensions of D 2 EHPA in benzene/aqueous solution systems were measured by the Wilhelmy plate method. Measurements were carried out fora metal ion-free system and for a system with cupric or ferric ion dissolved. In consequence, it is made clear that 1) the adsorption process is very rapid and the extractant can exist as a monomer or its dehydrogenized anion at the interface, 2) in cupric ion extraction systems, 1: 2 complexes are formed but no intermediate metal-ligand complexes can exist at the interface, and 3) in ferric ion extraction systems, 1: 2 intermediate complexes are formed and can adsorb with monomer extractants at the interface. The respective actual behaviors of the interfacial tension are shown to be explained well by Gibbs equation with the aid of the reaction model deduced from present experimental insights.