The use of extractive fermentation in penicillin production can overcome the low yield of penicillin. In this study, the liquid-liquid equilibrium (LLE) data of the ternary aqueous two-phase system (ATPS) of (polyethylene glycol 6000 + dipotassium hydrogen phosphate + water) at 298.2 K were measured. Then, penicillin G was added to the ternary ATPS and the LLE of the quaternary system at 298.2 K was measured. The maximum distribution coefficient of 15.63 was experimentally observed for penicillin G that indicates the good performance of the system in separating penicillin G from the fermentation broth. The experimental LLE data were modeled with the NRTL model with average mass fraction deviations of 0.004 for both ternary and quaternary systems. The model is employed to investigate the effect of mixture composition on the distribution coefficient of penicillin G. The results show that the decrease of water in the mixture increases significantly the distribution coefficient of penicillin G, while increasing K2HPO4 has a small positive effect on the distribution coefficient. (C) 2018 Elsevier Ltd.