The effects of nano-sized Pt deposits on TiO2 particles on the photocatalytic kinetics and mechanisms for the degradation of ammonia and trichloroacetate (TCA) in water and gaseous CO oxidation were investigated. Although platinization has been a routine method of TiO2 modification, its effects on photocatalytic reaction mechanisms in pollutant degradation need to be fully understood. TiO2 photocatalytic reactions are usually non-selective mainly because of the strong oxidation potential of OH radicals. (1) By using TiO2 deposited with nano-Pt particles, we are able to oxidize NH3 selectively to N2. While the pure TiO2 quantitatively transforms ammonia into nitrite and nitrate, Pt/TiO2 converts NH3 into N2 with reducing the total nitrogen concentration in the suspension. (2) TCA degrades photocatalytically in the presence or absence of dissolved O2, which proposes that two mechanistic pathways (oxic vs. anoxic) are operative. While the anoxic path is not important on naked TiO2, it becomes dominant on platinized TiO2 surface. Therefore, the introduction of dioxygen in aqueous suspension enhances the rate of TCA degradation on naked TiO2 but inhibits it on Pt/TiO2. (3) Platinum deposition on TiO2 greatly enhances the photooxidation rate of CO and the degree of enhancement strongly depends on the kind of TiO2 substrate. Platinized Hombikat TiO2 shows the best activity. The photocatalytic conversion of CO to CO2 is quantitative in the presence Of O2 and no CO2 is generated in the absence of O2.