This article reports on numerical simulations and experimental determination of the hydrodynamics of liquid-liquid flow in rectangular microchannels. The numerical method is an interface-capturing technique without any interface reconstruction. Micro-particle image velocimetry (micro-PIV) measurements are used to obtain experimental velocity fields inside droplets that are compared to simulations. Finally, injecting a passive tracer in the simulated droplets also helps to obtain a better understanding of the mixing phenomenon. These results allow interpreting mixing defaults during bleaching experiments inside microdroplets. Furthermore, this study leads to important results about interface deformation and velocity fields inside the droplets and in the continuous phase for mass- and heat-transfer studies. (c) 2006 American Institute of Chemical Engineers.