This study evaluates the influence of type of salt and temperature on the partition coefficient of caseinomacropetide (CMP) to determine the best conditions for the recovery of CMP in aqueous two-phase systems (ATPS) composed by poly(ethylene glycol) (PEG) 1500 and an inorganic salt (potassium phosphate, sodium citrate, lithium sulfate or sodium sulfate). In all systems, CMP presented affinity for the PEG-rich phase. The PEG1500 + lithium sulfate showed the highest values of partitioning coefficient. In addition, thermodynamic parameters (ΔH°, ΔS°, ΔG°) as a function of temperature, were calculated for the system PEG1500-sodium citrate at different PEG concentrations and the results imply thermodynamic differences between partitioning of CMP in this system. © 2007.