The partitioning behaviors of indium(III) and thallium(III) in polyethylene glycol (PEG)-(NH4)(2)SO4 aqueous two-phase systems (ATPSs) by 4-(2-pyridylazo) resorcinol (PAR) as an extractant have been investigated and have been compared to our previous work on gallium(III). The effects of acidity, PEG molecular weight, PEG concentration, and temperature on partition coefficients (K-p) have been discussed. Furthermore, thermodynamic parameters, such as enthalpy (Delta H degrees), Gibbs free energy (Delta G degrees), and entropy (Delta S degrees), for the partitioning process were also studied. The negative value of Delta G degrees and positive values of Delta S degrees and Delta H degrees indicated the spontaneous and endergonic nature for the gallium and indium partitioning process; on the contrary, the thallium process is exothermic. According to the results, PEG-based ATPSs have great potential for the removal and separation of gallium from indium and thallium by temperature and pH adjustments.