The interfacial tension of eight poly(ethylene glycol) (PEG) + salt aqueous two-phase systems, PEG+(NH 4 ) 2 SO 4 , PEG+K 2 HPO 4 , PEG+xK 2 HPO 4 +KH 2 PO 4 (x is the molar ratio of K 2 HPO 4 to KH 2 PO 4 , x=1.42, pH=7.0), PEG4000+xK 2 HPO 4 +KH 2 PO 4 (at different pH or x values), PEG+K 3 PO 4 , PEG+Na 2 SO 4 , PEG+MgSO 4 , and PEG+Na 2 CO 3 systems, is reported. The influence of polymer molecular weight and concentration, salt concentration and type, and the total solute compositions on the interfacial tension has been studied.