The self-motion of a sodium oleate (soap) disk at an oil/water interface was investigated as an autonomous system under nonequilibrium and isothermal conditions. The oleate molecules that developed from the disk transferred to both the toluene and water phases during self-motion. When the sodium oleate disk was removed from the toluene/water interface, the interfacial tension nearly recovered to the value without the disk. The duration of the motion increased as the volume of both phases increased. These results suggest that this self-motion continues due to a significant transfer of oleate molecules from the interface to both phases. (c) 2005 Elsevier B.V. All rights reserved.