The decomposition of the CoII complex of a macrocyclic cyclidene ligand is reported both in solution and when bound to a copolymer support. E.s.r. studies show that the decomposition of the complex in pyridine solution, under an atmosphere of O2, has an estimated half-life of ca. 36 h at 295 K. This suggests that decomposition of the O2 adduct is the second step observed in earlier electronic spectroscopic studies, and infers that the cyclidene O2 adducts are much longer lived than originally thought. The CoII cyclidene complex forms a coordinate bond to the pyridine group of n-butyl methacrylate/4-vinyl pyridine copolymers and the resulting five-coordinate complexes undergo reversible oxygenation. The polymer-supported O2 adducts are much more stable to decomposition than those in the solution phase and have an estimated half-life of ca. 30 days at 295 K. The enhanced stability to decomposition of the complexes in the polymer matrix is attributed to the low mobility of the complexes in the glassy polymer. © 1996 Chapman & Hall.