The interfacial tensions of two phase extraction systems, D 2 EHPA in benzene/ cupric aqueous solution and D 2 EHPA in benzene/ferric aqueous solution, were measured by a laser light scattering method. The interfacial tensions were confirmed to exhibit specific dependencies of extractant and hydrogen ion concentrations as had been determined by the Whilhelmy plate method. It was also verified that in the solution system without metallic ion, the monomer extractant and its anion are surface active, in a copper extraction system there forms no surface active intermediate complex, whereas in iron extraction system, there forms the 1:2 intermediate complex which is surface active.