The rate of mass transfer in the liquid—liquid extraction of the enzyme α-amylase between an aqueous phase and a reversed micellar phase has been investigated. Mass transfer rate coefficients have been measured in a mixer/settler and in a stirred cell. The pH of the aqueous phase determines the distribution coefficient of the enzyme and thus the direction of transfer. Forward transfer of the enzyme from the aqueous to the reversed micellar phase (cationic surfactant) occurs at pH 10.0. The mass transfer rate of this process was found to be controlled by the diffusion of the enzyme in the aqueous phase boundary layer. Back transfer from the reversed micellar phase to the aqueous phase occurs in the pH range 4–6. In contrast to the forward transfer, this process was found to be controlled by the interfacial process of enzyme release from the reversed micelles instead of the boundary layer diffusion. The same effects have been observed for the transfer of the enzyme ribonuclease A to and from a reversed micellar phase with an anionic surfactant. A mechanism to explain the different mass transfer behaviour in forward and back transfer is suggested.