The performance of microstructured reactors (or microchannels) for mass-transfer-controlled liquid-liquid reactions depends on flow regimes that define the specific interfacial area for the mass transfer. In the present work, experiments were carried out to investigate the two phase-flow regimes and the mass transfer at relatively high throughput for a single microchannel (of 1-18 mL/min) in five generic microchannel designs (with and without structured internal surfaces), using a nonreacting water-acetone-toluene system. When the flow results were analyzed collectively in all microchannels, six different flow regimes such as slug, slug-drop, deformed interface, parallel/annular, slug-dispersed, and dispersed flow were observed. The mass-transfer comparison shows that the microchannel with structured internal surfaces shows better performance, because it creates a very fine dispersion, providing high interfacial area, compared to other microchannels. Finally, the mass-transfer data were correlated, which can be used for a priori predictions of mass-transfer rates in microchannels.