A cationic hydrophobically modified ethylene oxide polymer(HM-EO)was used to form aqueous two- phase system with only one phase—forming composition．It was found that the phase separation could be achieved under room temperature．The phase behavior and charge properties of HM—EO aqueous two—phase systems were studied systematically．The amphiphilic property of HM—EO molecule resulted in the micellization and aggregation of miceLLes in water．The micellar aggregates formed in the bottom phase could be charged

and the net—charge could be adjusted by the pH and conductivity of bulk solution and adding sodium dodecyl sulfate(SDS)．The increase of pH would improve the phase separation of HM-EO aqueous two-phase system．The effect of salts on the phase behavior was Observed．The influence of cations varied as K+Na+

and that of anions as SO2-4>F->Cl->Br->I-．In addition．the interaction between HM-EO polymer and anion surfactant SDS was studied．The rosults indicated that the addition of SDS caused the formation of mixed miceHar aggregates of SDS and HM-EO which changed significantly the charge property of HM-EO aqueous two-phase systems.