The behavior of a water soluble calixarene ligand has been investigated in an (NH 4 ) 2 SO 4 /polyethylene glycol-2000 (PEG) aqueous biphasic system. Spectroscopic measurements indicate that 4-sulfonic-calix-6-arene partitions preferentially to the polymer-rich phase in this system. The utility of this ligand as a metal ion extractant was also investigated. The addition of the calixarene (10 - 2 M in initial PEG-rich stock solution) did not significantly enhance the partitioning of Na + , Cs + , Sr 2 + , Ba 2 + , Co 2 + , Cd 2 + , or UO 2 + 2 from the (NH 4 ) 2 SO 4 -rich to the PEG-rich phase. The partitioning behavior of these metal ions is dependent, rather, on their Gibbs free energy of hydration (ΔG h y d * ).