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 results on the phase equilibria of ATPS containing poly(ethylene glycol) (PEG) and either K2HPO4 or KH2PO4 or a fixed mixture of the two phosphates. The measurements were made at a temperature of 298.15 K and a pressure of 1 bar. The PEG had a mass-average molar mass of either 10 000 g center dot mol(-1) or 20 000 g center dot mol(-1) and a polydispersity index of 1.05. The influence of 2-hydroxy-propanoic acid (lactic acid) on the formation of the ATPS was studied, and the partition coefficient for this compound between the coexisting phases was found to be close to unity.