A series of charge-modified thaumatins with different values of surface charge were partitioned in aqueous twophase systems (ATPS) to study the effect of surface charge as a single property on partitioning. Electrophoretic mobility of the proteins in titration curves was used as a measure of surface charge. Four modified proteins derived from thaumatin with the following values of isoelectric point: 8. 70, 8.15, 5.60, and 4.50 were used for partitioning. The resolution of the systems in terms of protein surface charge was calculated. Partitioning of modified thaumatins in PEG 4000/dextran systems with phosphate buffer, Tris buffer, NaCI, KCI, and sulfate salts was carried out. Among the sulfate salts tested, the addition of 50 m M Li,SO, to the system buffered with phosphate gave the highest value of resolution for differences in surface protein charge (RSPC). It shows a decrease in the value of K (partition coefficient) with an increase in the protein's charge. The addition of 100 m M KCI to the system promoted the opposite effect on the RSPC value. Charge-modified proteins were partitioned in PEGlsalt systems to investigate the ability of these systems for resolving differences in surface charge. The PEG/citrate system seemed to have almost no ability for resolving proteins on the basis of surface charge differences; PEG/ phosphate systems had some capability for resolving differently charged proteins. The more negative proteins tended to have higher values of Kthan the more positively charged fractions. The use of charge-modified proteins allowed the investigation of the effect of protein surface charge on partitioning in aqueous two-phase systems independently from other protein parameters as they were prepared from a common parent protein thaumatin. This technique provides an interesting novel tool to investigate the effect of protein surface charge on partitioning in ATPS taking protein charge as an independent parameter.