Aqueous two-phase partitioning is one of the excellent bioseparation systems for proteins and other biornolecules in water-rich solutions containing polymers or salts. To improve the partition properties of model proteins, chicken egg white (CEW) proteins were chemically modified with polyethyleneoxide-maleic anhydride (PEOMA) copolymer. The modified proteins were partitioned in polyethyleneoxide-maleic acid/potassium phosphate aqueous two-phase systems. Firstly, the contribution of the chemical modification of proteins to the partition coefficient was investigated by using modified proteins with different degrees of modification (DM). The partition coefficients of modified proteins increased with increasing DM. The results suggested that the surface of the modified proteins had a greater hydrophobic property, similar to the PEOMA copolymer. Secondly, the effect of the pH on the partitioning of the modified proteins was studied. The partitioning of the modified proteins was found to be more dependent on pH in the alkaline systems. The increasing partition coefficient with pH at alkaline pH can be explained by the electrostatic repulsion between the phosphate anions and the maleic groups attached to the proteins. These results suggested that chemical modification of proteins with PEOMA could be a useful tool for separation and purification of crude protein mixtures. (C) 2005 Society of Chemical Industry.