A copolymer of 50% ethylene oxide and 50% propylene oxide (EOPO(1:1)) has been used in aqueous two-phase partitioning of amino acids. This EOPO(1:1) copolymer has a cloud point of 70 degrees C, at which it separates from water. The cloud point can be lowered to 45 degrees C by the addition of 0.2 M sodium sulfate. Amino acids are first partitioned at 25 degrees C in a system composed of EOPO(1:1) and hydroxypropyl starch (PES). EOPO(1:1) is enriched in the top phase and PES is in the bottom phase. Then, the top phase can be removed and its temperature increases above the cloud point of the EOPO(1:1). This results in formation of a new two-phase system with a top aqueous phase containing most of the amino acids and a bottom EOPO(1:1)-rich phase, thus allowing the lower EOPO(1:1)-rich phase to be removed and recycled for a second extraction. Amino acids are recovered exclusively in top aqueous phase.