During photocatalytic oxidation (PCO), methanol reacts to formaldehyde, some of which desorbs from TiO2 at room temperature. Formaldehyde that remains on the surface oxidizes to adsorbed formate, which dehydrogenates to CO2 in a single step without forming any long-lived intermediates. Adding trichloroethylene (TCE) during PCO of methanol increases the rates of methanol double right arrow formaldehyde and formic acid double right arrow CO2. Trichloroethylene decreases the rate that formaldehyde oxidizes to formate, however. Adding TCE during PCO of methanol, formaldehyde, and formic acid does not produce any new surface species or reaction pathways. Chlorine radicals that are produced during PCO of TCE may be responsible for the increases in dehydrogenation rates of methanol and formic acid.