Flow patterns in microstructured reactors (or microchannels) play an important role in dictating the mass transfer rates. In the present work, experiments were carried out to investigate the two phase (liquids) flow patterns in microchannels with different cross sections and contacting geometries. The pattern formation was analysed and conditions were classified in the three regions: surface tension dominated (slug flow), transition (slug-drop and deformed interface flow) and inertia dominated region (annular or parallel flow). A criterion for liquid-liquid flow pattern transition was developed using Capillary and Reynolds numbers based on the work of Dessimoz et al. Ill for gas-liquid systems. Finally, it was applied to the literature data and good agreement was obtained. The criterion is suitable for capillaries with hydraulic diameter up to 3 mm independently of cross section form and is an important predictive tool for the rational design of micro reactors for liquid-liquid reactions. (C) 2011 Elsevier B.V. All rights reserved.