The interface of a biphasic system of water and nitrobenzene exhibits a rapid flip motion and a tangential flow, so-called Marangoni convection, accompanying a pulse-like change of interfacial electrical potential when an aqueous solution of sodium alkylsulfate is introduced to the interface through a capillary. Although it is conventionally known that the Marangoni convection is triggered by heterogeneity of the interfacial tension, the heterogeneity of the interfacial tension of this system has not been directly measured. Here, we employed a time-resolved quasi-elastic laser scattering method with a time resolution of 200 ms to trace the time course of the interfacial tension at the water/nitrobenzene interface, and we demonstrated the first experimental evidence for the heterogeneity of the interfacial tension. Moreover, we found that the homogeneity of interfacial tension was restored within 1 s and that, in the presence of an electrolyte in the water phase, the surfactant molecules were homogeneously adsorbed on the interface immediately after they were conveyed by the tangential flow at the interface, whereas in the absence of electrolyte, the surfactant molecules were distributed only in the bulk phases. (C) 2010 Elsevier Inc. All rights reserved.