Four proteins with different physicochemical properties have been partitioned in reversed micelle systems : thaumatin, ribonuclease A, soybean trypsin inhibitor, and alpha-lactalbumin. The organic phase was formed by sodium salt (AOT) in isooctane, and the aqueous phase contained KCl, KBr, MgCl2, or NaCl. Aqueous phase pH was varied between 2 and 13 and ionic strength from 0.1 to 1.0 M. Small changes in pH [around the isoelectric point (pl)] were found to influence the solubilization of ribonuclease A and trypsin inhibitor, but for thaumatin the pH change necessary to affect partition was much greater as a consequence of the difference in net charge (titration curves) of these protein molecules as pH changes. The type of ions present in the system was also a determining factor for partition; the larger ions (K+) produced more electrostatic screening and hence less protein solubilization than the smaller ions (Na+). With changes in ionic strength surface hydrophobicity was a dominant factor affecting solubilization of thaumatin in NaCl-containing systems at high pH. Charge distribution and hydrophobicity are thought to be important parameters when partitioning the protein alpha-lactalbumin.