By means of micro-process engineering, the specific process-performance performance compared to conventional apparatuses can be increased. A model for the numerical simulation of the mass transport in a liquid-liquid plug flow in a micro-capillary reactor is presented. Following the idea of the interface-tracking method, separate computational domains for both phases are arranged around an imported steady-state interface topology. Experimental investigations are carried out using a standard extraction system. Around one third of the extraction takes place during the generation of the plug.