A new family of polymer conjugates is proposed to overcome constraints in the applicability of aqueous two-phase systems for the recovery of lactic acid. Polyethylene glycol-polyethylenimine (PEI) conjugates and ethylene oxide propylene oxide-PEI (EOPO-PEI) conjugates were synthesized. Aqueous two-phase systems were generated when the conjugates were mixed with fractionated dextran or crude hydrolyzed starch. With 2% phosphate buffer in the systems, phase diagrams with critical points of 3.9% EOPO-PEI-3.8% dextran (DEX) and 3.5% EOPO-PEI-7.9% crude starch were obtained. The phase separation temperature of 10% EOPO-PEI solutions titrated with tactic acid to pH 6 was 35 degrees C at 5% phosphate, a nd increased linearly to 63 degrees C at 2% phosphate. Lactic acid partitioned to the top conjugate-rich phase of the new aqueous two-phase systems. In particular, the lactic acid partition coefficient was 2.1 in 10% EOPO-PEI-8% DEX systems containing 2% phosphate. In the same systems, the partitioning of the lactic acid bacterium, Lactococcus lactis subsp. lactis, was 0.45. The partitioning of propionic, succinic, and citric acids was also determined in the new aqueous two-phase systems.