Proteins are spontaneously transferred from an aqueous solution into reversed micelles, provided the aqueous phase has the proper composition. Besides the composition of the aqueous phase, the composition of the organic phase and the properties of the proteins also play a role. We studied uptake profiles of 19 proteins as a function of pH of the aqueous solution. The organic phase consisted of trioctylmethylammonium chloride and nonylphenol pentaethoxylate (Rewopal HV5) as surfactant, octanol as cosurfactant and isooctane as continuous phase. In all cases, except for rubredoxin, proteins were transferred at pH values above their isoelectric point. The pH where maximal solubilization takes place can be described by the relationship: pHoptimum = isoelectric point f0.11 x 1O-j M , -0.97. So, the larger the protein, the more charge is needed to provide the energy required for the adaptation of the micellar size to the protein size. For protein transfer into sodium di-(2-ethylhexyl)sulphosuccinate Several fields of application for reversed micelles in biotechnology have been suggested (for reviews see [l -41). One of the potential applications is the extraction of proteins from an aqueous or solid phase via a reversed micellar phase into a second aqueous phase. This double transfer process is based on the ability of reversed micelles to extract proteins from an aqueous solution into their aqueous core. Most proteins retain their native conformation during the transfer process and remain in an active form when incorporated into reversed micelles [ 1 -61.Hatton and coworkers have shown that this technique is feasible for the extraction of proteins from a fermentation broth [7] and Leser et al. [8] demonstrated its applicability for the extraction of proteins from the solid phase. Previously [9] we reported that a-amylase could be extracted from an aqueous solution into a reversed micellar medium and re-extracted into a second aqueous phase in a continuous process with a yield of 45% and a concentration factor of eight with respect to enzyme activity. Addition of the nonionic surfactant nonylphenol pentaethoxylate (Rewopal HV5) to the organic phase, led to an increase both in the degree of solubilization of the enzyme and in the pH range in which solubilization occurs [lo]. By increasing the rate of mass transfer and the distribution coefficient of the enzyme during forward extraction, the yield has been improved to 85% and the concentration factor to 17 [ll].From the results published to date, it becomes evident that protein partitioning depends on the pH, ionic strength and type of ions present in the aqueous phase, on the type of surfactant, cosurfactant and organic solvent used, on micellar size, on the temperature and on properties of the protein [ 5 , 12 -161. However, no information is available on the mechanisms by which these variables affect protein partitioning. In this study the partitioning behaviour is related to such protein properties as size, isoelectric point and distribution of charged groups over th...