The effect of alcohol structure on photocatalytic production of H-2 from C-3 alcohols was studied on 0.5% Pt/TiO2. A C-2 alcohol (ethanol) was also included for comparative purposes. For individual reactions from 10% v/v aqueous solutions of alcohols, hydrogen production followed the order ethanol approximate to propan-2-ol > propan-1-ol > propane-1,2,3-triol > propane-1,2-diol > propane-1,3-diol. The process was found to be quite sensitive to the presence of additional alcohols in the reaction medium, as evidenced by competitive reactions. Therefore, propan-2-ol conversion was retarded in the presence of traces of the other alcohols, this effect being particularly significant for vicinal diols. Additional experiments showed that adsorption of alcohols on Pt/TiO2 followed the order propane-1,2,3-triol > propane-1,2-diol > propane-1,3-diol > propan-1-ol > ethanol > propan-2-ol. Adsorption studies (DRIFT) and monitoring of reaction products showed that the main photocatalyzed process for propan-2-ol and propan-1-ol transformation is dehydrogenation to the corresponding carbonyl compound (especially for propan-2-ol both in the liquid and the gas phase). In the case of liquid-phase transformation of propan-1-ol, ethane was also detected which is indicative of the dissociative mechanism to lead to the corresponding C-1 alkane. All in all, competitive reactions proved to be very useful for mechanistic studies.