Some processes in chemical technics are connected with transport of reactands through the interface between immiscible liquid phases. The effectivity of separation processes depends on thermodynamic conditions, kinetic processes at the interface and hydrodynamic conditions. A modified stirring cell (Lewis-type) for kinetic measurements has been developed and tested. Transfer coefficients of the transport of toluene into water at the system toluene/water and of carbonacids und benzeneseleninic acid at different pH of the same liquid-liquid system have been determined.