The capillary-microreactor was used for studying the nitration of a single ring aromatic in an exothermic liquid-liquid two-phase reaction. In the capillary-microreactor, isothermal behaviour can be assumed due to the high heat transfer rates. In addition. a liquid-liquid two-phase plug-flow with a well-defined flow pattern of alternating plugs of the two phases is formed, giving a constant, uniform specific surface area for mass transfer between the two phases. In the nitration reaction, not only the mononitrated main product is formed, but also various by-products via consecutive and parallel reactions. Mass transfer experiments with different flow velocities, but identical residence times, carried out in the capillary-microreactor, yielded differences in conversions and the amounts of by-products formed. Simulations using a mathematical model describing interphase mass transfer and homogeneous chemical reaction indicate increasing mass transfer coefficients at higher flow velocities, suggesting that the mass transfer between the two phases is enhanced by the flow velocity. The enhancement of the mass transfer can be interpreted in terms of an internal circulation flow within the plugs, a conclusion corroborated by CFD calculations. The capillary-microreactor has thus proved to be a useful instrument for the quantitative elucidation of the mechanisms of exothermic liquid-liquid two-phase reactions. (C) 2003 Elsevier Science B.V. All rights reserved.