In this paper, which forms part of a study on aqueous two-phase systems (ATPS) for separation of low molar mass organic acids in the biotechnology industry, we present experimental and modeling results on the phase equilibria of an ATPS containing poly( ethylene glycol) ( PEG) and poly( ethyleneimine) (PEI). The measurements were made at a temperature of 25 C and a pressure of 1 bar, at pH) 5.3, 7.5, and 9.2. The PEG had a mass-average molar mass of 4000 g . mol(-1) and a polydispersity index of 1.05, while the PEI had a mass-average molar mass of 25 000 g . mol(-1) and a polydispersity index of about 2.5. A UNIQUAC model, incorporating the polydispersity of the PEI, was found to give a satisfactory correlation of the experimental data. The experiment results demonstrated that the polymers are distributed unevenly in the two phases, and this suggests that organic acids ( e. g., lactic acid) will partition preferentially to the PEI-rich phase through acid-base association.