Aqueous two phase systems (ATPS) offer great potential for selective separation of a wide range of biomolecules by exploring differences in solubility in each of the two phases. However, their use has been greatly hindered due to poor theoretical understanding of the principles behind ATPS formation and the empirical and time-consuming techniques used for the determination of optimal extraction parameters including the binodal curves. In this work, characteristic ATPS binodal curves were determined by a novel technique in which the formation of an ATPS system is measured in a microfluidic device. Two solutions containing separate ATPS solution precursors were loaded into the side inlets of a three inlet microfluidic channel while milli-Q water was loaded into the middle inlet. By varying the flow rates of the three solutions, a wide range of concentrations inside the microchannel could be rapidly tested using limited volumes. Using optical microscopy, depending on the concentrations inside the microchannel, three different states could be observed at the end of the microchannel (i) the presence of an interface; (ii) no presence of an interface; or (iii) the presence of an unstable interface. The binodal curve was calculated using the points corresponding to unstable interfaces and compared to binodal curves obtained through the standard turbidometric titration method for both PEG/salt and PEG/dextran systems.