VIRTUALLY all synthetic materials with a dynamic function, from catalysts to integrated circuit elements, degrade irreversibly with time. The inevitability of decay is an implicit consideration in the design of materials or molecules that serve these functions, and fabrication methods tend to aim simply at minimizing the rate of decay. Here, by contrast, we describe a molecular catalyst that experiences a thermodynamic and kinetic driving force for its own reassembly and repair under the conditions of catalysis. We show that the multicomponent polyanion cluster alpha-[(Co-11)PW11O39](5-) self-assembles from four precursor species, containing a total of 28 molecules, and that as it assembles it starts simultaneously to catalyse the epoxidation of alkenes with high selectivity. This conclusion follows from the observation that the kinetics of self-assembly and those of catalysis are closely correlated as the reactions proceed. Should it be fragmented during operation, this polyanion catalyst will therefore experience a thermodynamic and kinetic driving force for its own repair.