Transient, isothermal photocatalytic oxidation (PCO) was combined with isotope labeling and temperature-programmed desorption and oxidation to directly identify reaction pathways and intermediates for the room-temperature PCO and ethanol on TiO2. The intermediates identified are acetaldehyde, acetic acid (acetate), formaldehyde, and formic acid (formate). The alpha-carbons of ethanol, acetaldehyde, and acetic acid were labeled with C-13 so that the reaction pathway of each carbon could be followed. For each molecule, the alpha-carbon preferentially oxidized to CO2 as the two-carbon species were sequentially oxidized. Ethanol forms acetaldehyde, which either desorbs or oxidizes through at least two parallel pathways, only one of which involves acetic acid. Part of the ethanol reacts on the surface through the pathway : acetaldehyde --> acetic acid --> CO2 + formaldehyde --> formic acid --> CO2. The remaining ethanol oxidizes more slowly through a pathway that does not contain acetic acid as an intermediate : acetaldehyde --> formic acids + formaldehyde --> formic acid --> CO2. The oxidation of ethanol to acetaldehyde is not the rate-determining step. The oxidations of formaldehyde to formic acid, and formic acid to CO2, occur at about the same rate, which is faster than acetic acid oxidation. Acetaldehyde oxidizes to form intermediates at approximately the same rate as they are oxidized. The presence of acetaldehyde on the surface, however, decreases the reactivity of other intermediates, suggesting that increasing the rate of acetaldehyde oxidation would increase the overall rate of CO2 production.