The effects of nanosized platinum deposits, dioxygen, and electron donors on the kinetics and mechanism of trichloroacetate (TCA) degradation in UV-illuminated TiO2 suspensions are described. Although platinization has been a routine method of TiO2 modification, its effects on photocatalytic reaction mechanisms in pollutant degradation need to be fully understood. By selecting TCA as a model compound, we demonstrated that the presence of Pt deposits (typical diameters of 1-2 nm) on TiO2 markedly changed its mechanism of photocatalytic degradation. TCA degraded photocatalytically in the presence or absence of dioxygen, which proposed that two mechanistic pathways (oxic vs anoxic) were operative. While the anoxic path was not important on bare TiO2, it became dominant on the platinized TiO2 surface. It is suggested that the platinum surface stabilized the reactive intermediates (e.g., dichlorocarbene) and subsequently changed the mechanistic pathway of TCA degradation. Therefore, the introduction of dioxygen in aqueous suspension enhanced the rate of TCA degradation on bare TiO2 but inhibited it on Pt-TiO2. The negative effect of dioxygen was much greater with platinum-rutile than platinum-anatase. The addition of excess methanol or tert-butyl alcohol as a hole scavenger, however, made the oxic path more favored on both bare and platinized TiO2.