Dipeptides are partitioned in aqueous two-phase systems containing Poly(ethylene glycol) and dextran. NaCl and Na2SO4 are added to the system with a concentration of 40 mmol/kg. The pH of the system is adjusted with HCl or NaOH directly to keep the concentrations of the electrolytes to a minimum. By partition experiments with a charged and an uncharged test substance it is shown that neither the basic two-phase system nor the electrostatic potential difference between the coexisting phases is influenced by the electrolytes in the pH-range studied. The results show that the model proposed by Albertsson (1986) can successfully be applied to the partition behaviour of proteins over the entire pH-range investigated, if their acid-base equilibria are taken into account appropriately. This is done by assuming that the differently charged subspecies of each dipeptide partition independently of each other and that the equilibria between these subspecies can be described based on their pK(i)-values. Simultaneously the difference between the electrostatic potential differences, DELTAphi(Na2So4) - DELTAphi(NaCl, has been unambiguously determined to be 4.8 +/- 0.5 mV for the system chosen.