The development of a procedure for the epoxidation of alkenes with molecular oxygen is an important industrial and synthetic goal. Non-radical activation of dioxygen by monooxygenase enzymes such as the iron-porphyrin-based cytochrome P-450 involves the insertion of one oxygen atom of O-2 into the organic substrate while the other oxygen is reduced to water in the presence of an electron donor. Dioxygenase enzymes, on the other hand, catalyse insertion of both oxygen atoms without reducing agents. Oxidation of hydrocarbons with dioxygen catalysed by transition-metal compounds invariably proceeds by free-radical pathways rather than by dioxygenase-type reactions, and so do not allow the epoxidation of alkenes with molecular oxygen. The sterically hindered ruthenium tetramesityl porphyrin complex has been shown previously to activate dioxygen in a dioxygenase mode(1). We describe here the use of the polyoxometalate {[WznRu(2)(OH)(H2O)](ZnW9O34)(2)}(11-) (ref. 2) as a catalyst for non-radical molecular oxygen activation and alkene epoxidation. The polyoxometalate can be considered to act as an inorganic dioxygenase catalyst. The advantages of using inorganic catalysts such as polyoxometalates, as opposed to organometallic complexes, is their well documented stability against decomposition by self-oxidation(3,4).