A theoretical description and an experimental validation of the application of a microfluidic chip with high-velocity stratified flows for determining chemical kinetics for liquid-liquid extraction are presented. In the case of uranium extraction under PUREX (plutonium and uranium refining by extraction) process conditions, a simple theoretical model demonstrates the need for high velocities and short residence times of around 10 ms. Confocal microscopy observations of the interface were undertaken to insure the flow stability at such high velocities, and the same experimental protocol was carried out to uranium(VI) extraction at two concentrations. Results show an unexpected variation in the phase homogenization depending on the uranium concentration of the extracted phase.