The flow regimes are identified and mass transfer rates measured in a serpentine micro-reactor with a hydraulic diameter of 0.7 mm using the two-phase alkaline hydrolysis of 4-nitrophenyl acetate between flow rates of 0.8 and 32 mL/min. Four different organic solvents (n-butanol, n-hexanol, MTBE and toluene) were used in order to investigate the effect of phase physical properties. The continuous phase overall volumetric mass transfer coefficient, K(c)a, was greatest in the drop flow regime and for organic solvents with lower interfacial tension resulting in larger interfacial area, a. In the parallel flow regime, K(c)a values increased linearly with the continuous phase capillary number. In the drop flow regime, two correlations for the prediction of the drop size were developed with fitted parameters falling within the expected values for turbulent break-up of drops in conventional static mixers. (C) 2015 Elsevier B.V. All rights reserved.