Commercial interest in cost-effective methods that can separate, concentrate, and purify proteins continuously, and be easily scaled-up, has increased markedly in recent years. Liquid-liquid extraction using reverse micellar organic solvents has been found to have this potential. The overall objective of this work was to investigate the kinetic separation of protein mixtures using reverse micelles. The kinetics of simultaneous forward extraction of lysozyme, cytochrome c, and ribonuclease A were investigated in a stirred cell (Lewis cell). In addition, a Graesser contactor was used for the simultaneous extraction of lysozyme and cytochrome c from a buffer solution as well as from diluted hen egg white to which cytochrome c had been added. The Lewis cell experiments showed that the mutual effect on the overall mass transfer coefficient, k(0), of the three simultaneously extracted proteins at pH 5.5 was significant, and in some cases quite large. The presence of cytochrome c or ribonuclease A helped to prevent lysozyme precipitation at the interface, while the addition of cytochrome c altered the pH dependency pattern of lysozyme, and the presence of lysozyme reduced the k(0)-value of cytochrome c. The kinetic separation of lysozyme and cytochrome c was possible with a Graesser contactor, and maximized at low rotor speed (2-3 rpm), low temperature (4 degrees C), and a pH close to the pI of both proteins (pH 10); after 30 minutes about 80% of the lysozyme and only 10% of the cytochrome c were extracted into the reverse micellar phase. The extraction rate measured in the Graesser contactor differed from that measured in the Lewis cell, and this observation indicates that different steps of the reverse micellar transfer mechanism are controlling the transfer. Using hen egg white (the natural source of lysozyme), the kinetics of lysozyme and added cytochrome c were different from those with buffer solutions; cytochrome c extraction was faster. making kinetic separation more difficult. In addition, a stable emulsion formed with hen egg white, which is not desirable in an industrial application since it results in an additional separation step.