We investigated the surface (photo)chemistry of alcohols on TiO2(110) and recently Fe2O3(012) single crystals in ultra-high vacuum. Our studies focused on the role of the metal co-catalyst in the photocatalytic reaction by comparing the reactivity of bare and metal-decorated surfaces. We show that photocatalytic reactions are not merely a couple of redox reactions, but an interplay of thermal and photon-driven steps. On TiO2(110), the alcohol photoconversion allows for a steady-state production of molecular hydro- gen and aldehydes/ketones. We rationalize the findings with a comprehensive mechanism taking into account stoichiometry and charge-carrier dynamics. The identification of surface mechanisms on Fe2O3 is less advanced, but there seem to be parallels in the photochemistry