Figure 1: (a) Schematic of molecule mass transfer across the two immiscible phase boundary. (b) Schematic of the solute extracting (concentration varying) process indicated by droplets at different reaction times. ABSTRACT This paper demonstrated a fluorescence-indicated mass transfer liquid-liquid extraction process based on a droplet microfluidic system. Analysis of the droplet-based liquid-liquid extraction process based on the description of mass transfer into or out of droplets is conducted. The relationship between the extraction efficiency and the droplet size is measured and the mass transfer coefficient for fluorescein from octanol to PBS buffer is determined as 3.07 × 10 -5 m/s. This work has wide potential applications in extraction system design and other chemical-biological areas.