The reactions of ethanol have been investigated over anatase TiO₂(101) and rutile TiO₂ (110) single crystals by STM and on-line mass spectrometry to determine the adsorbate species in the dark and post UV illumination, in the presence and absence of O₂ or Au nanoclusters, in order to extract initial reaction parameters under photo-excitation. On anatase (101) TiO₂ single crystal the reaction rate for the photo-oxidation of ethanol to acetaldehyde is found to be strongly dependent on O₂ partial pressures and surface coverage with an order of the reaction for O₂ close to 0.15. Carbon-carbon bond dissociation leading to CH₃ radicals in the gas phase was found to be a minor pathway, which is contrary to the case of TiO₂ rutile (110) single crystal, as previously reported by our group and others. Our STM images distinguished two types of surface adsorbates upon ethanol exposure that can be attributed to its molecular and dissociative modes. Upon UV exposure at (and above) 3×10^-8 mbar O₂, a third species is identified as a reaction end-product, which can be tentatively attributed to acetate/formate species, in line with XPS C1s measurements. The room temperature photo-oxidation of ethanol has also been investigated over a rutile TiO₂(110) single crystal by STM and on-line mass spectrometry, in the presence of O₂ to determine adsorbate species remaining post UV exposure and emitted gas phase products. In addition to acetaldehyde, methyl radical was detected in the mass spectrometry resulting from the photo-fragmentation of an acetaldehyde-O complex. Formic acid species bound to Ti_5c in a bi-dentate mode were identified by STM on the surface after the reaction. The effect of O₂ partial pressure on the reaction selectivity demonstrated a dominance of the photo-fragmentation process with increasing pressure unlike the case of anatase TiO₂(101) single crystal. In addition the effect of Au particles on the reaction of the TiO₂(110 rutile has been investigated for hydrogen production under photo-irradiation in UHV conditions. It was found that particle size in the range 4 to 8 nm do not change the reaction rate yet particle density does. Detailed reaction mechanism for both reduction and oxidation reactions are addressed based on structural and kinetic information from the UHV systems and compared to those of their corresponding powder forms in liquid-solid photocatalytic reactions conditions.