This report is about microfluidic extraction systems based on droplets of aqueous two-phase system. Mass transfer between continuous phase and dispersed droplet is demonstrated by microextraction of ruthenium red in a microfluidic device. Droplets are generated with electrohydrodynamic method in the same device. By comparing brightness in the digital image of a solution with known concentrations of ruthenium red and those of a droplet in the microextraction, ruthenium red concentration was measured along the microextraction channel, resulting in good agreement with a simple diffusion model. The maximum partition coefficient was 9.58 in the experiment with the 70-mm-long-channel microextractor. The method is usable for terminating microextraction by electrohydrodynamic manipulation of droplet movement direction. Droplets of different ruthenium red concentration, 0.12 and 0.24% (w/w) in this experiment, can be moved to desired place of microfluidic system for further reaction through respectively branched outlets. In this study droplet-based microextraction is demonstrated and the mass transport is numerically analyzed by solving the diffusion-dissolution model. (C) 2010 Elsevier B.V. All rights reserved.