Synthetically useful hydrocarbon oxidations are catalysed by bio-inspired non-heme iron complexes using hydrogen peroxide as oxidant, and carboxylic acid addition enhances their selectivity and catalytic efficiency. Talsi has identified a low-intensity g=2.7 electron paramagnetic resonance signal in such catalytic systems and attributed it to an oxoiron(V)-carboxylate oxidant. Herein we report the use of FeII (TPA*) (TPA*=tris(3,5- dimethyl-4-methoxypyridyl-2-methyl)amine) to generate this intermediate in 50% yield, and have characterized it by ultraviolet-visible, resonance Raman, Mössbauer and electrospray ionization mass spectrometric methods as a low-spin acylperoxoiron(III) species. Kinetic studies show that this intermediate is not itself the oxidant but decays via a unimolecular rate-determining step to unmask a powerful oxidant. The latter is shown by density functional theory calculations to be an oxoiron(V) species that oxidises substrate without a barrier. This study provides a mechanistic scenario for understanding catalyst reactivity and selectivity as well as a basis for improving catalyst design.