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Despite the proven properties of the anatase phase of TiO₂ related to photocatalysis, detailed mechanistic information regarding a photooxidation reaction has not yet been derived from single-crystal studies. In this work, we have studied the photooxidation of ethanol (as a prototype hole-scavenger organic molecule) adsorbed on the anatase TiO₂(101) surface by STM and online mass spectrometry to determine the adsorbate species in the dark and under UV illumination in the presence of O₂ and to extract kinetic reaction parameters under photoexcitation. The reaction rate for the photooxidation of ethanol to acetaldehyde was found to depend on the O₂ partial pressure and surface coverage, where the order of the reaction with respect to O₂ is close to 0.15. Carbon–carbon bond dissociation leading to the formation of CH₃ radicals in the gas phase was found to be a minor pathway, which is contrary to the case …