Quaternary (liquid + liquid) equilibrium (LLE) data of the aqueous two-phase poly (ethylene glycol), poly (N, N-dimethylacrylamide-t-butylacrylamide) with abbreviation name poly (DMAM-TBAM) as a hydrophobic association water-soluble copolymer and KH2PO4 has been determined experimentally at T = 338.15 K. Furthermore, the generalized Flory-Huggins theory with two electrostatic terms (the Debye-Huckel and Pitzer-Debye-Huckel) was used for correlation of the phase behavior of the quaternary system and the interaction parameters between all species were calculated. It was found that addition of poly (DMAM-FBAM) copolymer as well as changing the temperature can shift the binodal curves of aqueous two-phase systems containing polyethylene glycol (PEG) and salt. Also, the phase behavior of the DMAM-TBAM copolymer with sonic salts containing sodium chloride, ammonium hydrogen phosphate, potassium hydrogen phosphate, and sodium carbonate were studied experimentally at T = 338.15 K and the effect of the salt type on the their binodal curves was determined. (C) 2009 Elsevier Ltd. All rights reserved.