A visualization technique based on microscope and photoelectric has been developed to investigate phase behaviour and interfacial phenomena at immiscible liquids interface on micron scale. An imaging analysis software based on advanced imaging techniques was also established for determination of the relationship between the intensity of the components and their concentration. The processes of mass transfer across the interface of two inmiscible liquids were investigated for paraffin-water, ethychloride-water, ethylbenzene-water, diphenyl ether-water and dichloro methane-water, respectively. The result shown that there was a dynamic interface between two iquids bulk phases for all studied systems. The boundary between bulk phase and interface phase is sharp and clear, but not smooth everywhere on the micro scale. A lot of micro micelles which random moved in the convection region were observed, although the system was not agitated by forced convection. The rotating and vibrating of the micelles lead to microcirculation flows spreading to transition region from bulk phase. However, they did not pass through the interface. The thickness of interface phase was also measured in the range of approximate ten microns by the sub-micro visualization technique. Compared with Hansen's and Cahu-Hilliard's theories about thermodynamic thickness of interface phase, the dynamic thickness of interface phase was observed for a certain period of time. The behaviour of the interface phase observed in the experiments suggests that the main reason for high selectivity in separation processes is the special structure of interface phase.