In this work, the partition coefficients of Cephalexin in aqueous two-phase systems containing PEG [poly(ethylene glycol)] 4000, 10000 and K2HPO4, Na(3)Citrate (C6H5Na3O7 5, 5H(2)O) have been measured. The experimental data were obtained in a wide range of temperatures, (28.2 to 37.2) degrees C. The effects of temperature, pH, polymer concentration, polymer molecular weight, and salt concentration on the partitioning of Ceohalexin were also studied. The results showed that salt concentration has a large effect on the partition coefficient, and temperature has an almost negligible effect. The Chen-NRTL Gibbs energy model was used to Correlate the experimental results.