Methyl formate adsorbs both molecularly and dissociatively as methoxy and formate on titanium dioxide (TiO2). Methyl formate dissociates on TiO2 at dual sites that adsorb methoxy and formate; formate adsorption sites are more numerous than those of methoxy and the availability of methoxy sites limits methyl formate coverage. Photocatalytic oxidation (PCO) oxidizes the cl-carbon in methyl formate to CO2 without forming any long-lived intermediates, whereas the beta-carbon forms CO2 through formaldehyde and formate. 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 adsorption sites. Photocatalytic oxidation carried out at 373 K oxidizes species more quickly than at room temperature because elevated temperatures enhance inherent site activity and convert adsorbed methoxy to formate. Water does not poison methyl formate PCO and redistributes weakly bound species during PCO by displacement. Water preferentially displaces methanol and also converts some adsorbed methoxy to formate at room temperature. (C) 2004 Elsevier Inc. All rights reserved.