In this Letter, we show the benefit of using a lyotropic lamellar phase as a self assembled model system to investigate the distribution of a lipophilic molecule at a hydrophilic/hydrophobic interface. For complexing agents used for ion transfer in liquid/liquid extraction it is essential to determine their interfacial activity. Coupling scattering and spectrometry techniques, we show that it is possible to determine accurately the amount of this type of molecule at the interface. (C) 2010 Elsevier B. V. All rights reserved.