The equilibrium interfacial tensions at the hydrocarbon/water interfaces, as measured at 20 degreesC for copper(II) complexes with the chelating ligands beta-diketone (1-phenyl-1,3-decanedione), 2-hydroxy-5-t-nonylacetophenone oxime, 2-hydroxy-5-t-octylacetophenone and 2-hydroxy-5-t-octylbenzophenone oxime, as well as for individual extractants, are discussed. The interfacial tension and surface excess isotherms were determined and interpreted. The experimentally obtained curves were Successfully matched with the Szyszkowski and Frumkin equations. It was found that the tendencies of interfacial tension to be lower at the hydrocarbon/water interfaces for copper(II) complexes and individual chelating ligands are similar. However, the effectiveness and efficiency of adsorption for chelatin, extractants and copper(II) complexes are quite different, irrespective of the type of hydrocarbon used as the organic diluent. On the other hand, the type of organic phase affects the interfacial activity of the investigated compounds. Moreover, it was shown that the way of preparing the measurement systems with the copper(II) complex influences the results of its interfacial activity. (C) 2004 Elsevier Inc. All rights reserved.