Surface tension measurement at an aqueous drop/1,2-dichloroethane interface was used to study adsorption and surface reactions of calix[4]arene ligands, The found difference in surface tension in acidic and alkaline medium was ascribed to a difference in charge of the adsorbed ligand. Formation of a surface complex with Ba2+ and Ca2+ cations reduces the change in surface tension as compared to an adsorbed dissociated ligand with higher charge. A mechanism of the surface reaction is proposed. (C) 1998 Elsevier Science Ltd. All rights reserved.