Hematite is a promising catalyst for the photoelectrochemical water oxidation reaction, which however displays a low overall efficiency. To improve it, a systematic understanding of the underlying photocatalytic mechanisms is desirable but difficult to obtain by experimental techniques alone. We have investigated the oxidation of water on the most stable terminations of the bare hematite by first-principles density functional theory-based methods. Although several surface terminations are very close in surface energy, they all yield a very similar overpotential of ∼0.8 V. Moreover, on all the relevant terminations, the overpotential-determining reaction step is the same, involving the dehydrogenation of a surface-adsorbed hydroxyl species. The reaction mechanism is largely independent of the atomistic details of the surface termination and crucially involves the formation of reaction intermediates involving lattice oxygen bound to adsorbed oxygen from water (O*−Os). It seems likely not only that different surface terminations coexist but also that they transform into one another during reaction conditions, even at the steady state. We also shed light on the important role of midgap states of hematite in the water oxidation cycle. In presence of the O*−Os species, midgap states that localize on this species can act as long-lived hole traps and fall prey toparasitic recombination processes. Without the O*−Os species, the surface states of hematite compete with the states on the active water oxidation species to attract the holes.