The kinetics of protein partitioning have been determined for the batch extraction of proteins using reverse micellar phases comprised of AOT in isooctane. Experiments were carried out in well-mixed systems and a mathematical model, based upon the two-film theory of mass transfer, allowed quantitative description of the transfer kinetics. Forward transfer of the protein, into the reverse micelle phase, was favoured at low pH (< pI) and low ionic strength where attractive electrostatic interactions existed between the protein and the micelles. The rate of the forward transfer process increased with decreasing pH and ionic strength whilst the opposite conditions (i.e. high pH and ionic strength) favoured the rapid back extraction of protein from the reverse micellar phase. The kinetics of the back extraction, however, were approximately two orders of magnitude slower than for the forward extraction. Protein partitioning kinetics were generally similar at volume ratios (V-r = V-aq/V-rm) of V-r = 1 and V-r= 5. Differences were found concerning the partitioning behaviour of the proteins investigated, lysozyme and ribonuclease-a, which were attributed to hydrophobic interactions occurring between the protein and the micelle wall. However, by alteration of the extraction pH, it was possible to separate a mixture of the two proteins based upon both their equilibrium partition and also the kinetics of extraction. Both proteins were found to retain 80-100% of their activity after undergoing the extraction process.