The ability of certain Fe and Mn metalloporphyrins to catalyse the H2O2 epoxidation of cyclooctene is analysed. The 'efficiency' of epoxidation is dissected into contributions due to inherent catalyst epoxidation ability, catalyst H2O2 dismutation ability, and catalyst stability towards the oxidant. It is shown that catalyst stability is a major factor contributing to apparent catalyst ability. The preparation of a sol-gel encapsulated metalloporphyrin is reported and it is shown to exhibit reduced rate of epoxidation, but a much enhanced stability. (C) 2002 Elsevier Science B.V All rights reserved.