Photocatalytic hydrogen production from water or oxygenates is foreseen as a sustainable energy production route. In spite of recent performance achievements through Pt particle downsizing, the knowledge on the prototypical Pt/TiO 2 photocatalyst operation mechanism, in particular the electronic state and the stability of the Pt phase under reaction conditions, remains limited. We have investigated atomically dispersed Pt/TiO 2 catalysts by pre/post-reaction scanning transmission electron microscopy and operando X-ray absorption spectroscopy-mass spectrometry in gas-phase ethanol dehydrogenation conditions under ultraviolet-visible light irradiation. The catalysts were prepared by a simple impregnation method on two types of commercial titania. While the general effect of the reaction on initially fully oxidized (Pt IV) single-atom catalysts (SACs) is Pt reduction and aggregation, this combined phenomenon can be inhibited in two manners. First, when supported on high-surface-area anatase, the Pt SAC retains an intermediate oxidation state, and its clustering is limited to Pt dimers and trimers. Second, with the same support, a mild reducing pretreatment generates, through the formation of Pt-Ti bonds, near-neutral and ultradispersed Pt species (from single atoms to nanometric clusters) with the highest stability and activity in photocatalytic hydrogen evolution.