The kinetics of solute extraction in sodium bis(2-ethylhexyl)sulfosuccinate (AOT) water-in-oil microemulsion/brine two-phase system was elucidated. The rate process of the tryptophan extraction was examined by measuring the time evolution of the concentration profiles of tryptophan near the liquid/liquid interface in a static diffusion cell and a flow junction cell. An anomalous phenomenon was observed for the solute transfer when water was coextracted to the microemulsion phase, whereas the anomaly disappeared when using the microemulsion presaturated with water. The interfacial rate process played an essential role in the extraction of solute with the presaturated microemulsion. The effects of some key factors on the interfacial rate process were examined in AOT/n-heptane/NaCl-brine system using a stirred transfer cell. These factors were the electric charge states of solutes, the location of the solute entrapped in a microemulsion droplet, the direction of extraction, the diameter of a droplet, and the surfactant concentrations. Organic solvent had a great effect on the interfacial rate process, indicating that the fusion process between a microemulsion droplet and the liquid-liquid interface is the rate-determining step. The rate of extraction was formulated based on the rate-limiting fusion process and the fusion rate constant was determined.