The flow regimes and mass transfer rates in five complex micro-reactors with different mixing mechanisms were investigated using the two-phase alkaline hydrolysis of 4-nitrophenyl acetate. n-Butanol and toluene were used as organic solvents. Using n-butanol in curvature-based micro-mixers, the flow regime evolved from slug to parallel to drop/dispersed flow with increasing flow rates. In obstacle based micro-mixers, no parallel flow was observed. Using toluene, no parallel flow was observed for all reactors. The conversion of 4-nitrophenyl acetate was found to be strongly dependent on the flow regime. In slug and parallel flow, the conversion generally decreased with an increase in flow rate whereas it typically increased in drop flow and was constant or slightly decreased in dispersed flow. The different micro-mixers were compared using the overall volumetric mass transfer coefficient, K(org)a, which was primarily a function of the rate of energy dissipation within the dispersed flow regime. The geometry itself impacts the resulting flow regime and rate of energy dissipation at a given flow rate. The micro-reactors were then compared using modified Damkohler's numbers. Curvature-based reactors were found to be inadequate for liquid-liquid reactions under the studied conditions, as they favor parallel flow patterns and yield relatively low interphase mass transfer rates. (C) 2016 Elsevier B.V. All rights reserved.