Polymer-supported cobalt(II) ＇lacunar＇ complexes undergo reversible oxygenation with enhanced stability to autoxidation. The use of copolymer supports containing a donor atom capable of covalently binding to the metal centre allows production of solid state systems that display reversible binding of O-2 With a half life of the order of 30 days. However, these systems do not function as facilitated transport membranes for enhanced dioxygen separation. Scanning electron microscopy studies indicate this is because the copolymer/complex films show phase separation arising from the charged nature of the lacunar complexes.