Separation of globular proteins (cytochrome c and ribonuclease A) from buffer using precipitation by sodium bis-(2-ethylhexyl) sulfosuccinate (AOT), and their recovery with a counterionic surfactant, trioctylmethylammonium chloride (TOMAC) was investigated. The molar ratio between AOT and the protein (R) required for complete removal was 17 for cytochrome c, and 22 for ribonuclease A. Finally, three mixtures of the two proteins and lysozyme (studied earlier by us) were used to determine the factors controlling separation: selectivity was a strong function of surface charge distribution, indicating that charge interactions between the surfactant and surface groups of different proteins was driving precipitation.