Photocatalytic oxidation (PCO) quickly oxidizes dichloroacetyl chloride and dichloroacetic acid to phosgene (COCl2) and CO2 on TiO2. At least two types of active sites exist for PCO on TiO2 and their activities differ by more than an order of magnitude; the more-active sites comprise approximately 30% of the adsorption sites. Water redistributes adsorbed dichloroacetyl chloride and dichloroacetic acid to more-active sites during transient PCO, yet does not change PCO activity or selectivity. Dichloroacetyl chloride oxidizes through parallel pathways, one of which forms dichloroacetic acid as an intermediate. The alpha-carbon in dichloroacetic acid quickly oxidizes to CO2 without forming any long-lived intermediates, whereas the beta-carbon forms CO2 and COCl2 in parallel reactions.