The role of Pt on photocatalytic substrates such as TiO2 (P25) for the decomposition of organic pollutants is still controversial in the scientific community. The well-observed behavior of an optimum catalytic activity as a function of the Pt loading is usually explained by the shift from charge separation to charge recombination behavior of Pt clusters. However, experiments supporting this explanation are still lacking to give a concise understanding of the effect of Pt on the photocatalytic activity. Here, we present an experimental study that tries to discriminate the different effects influencing the photocatalytic activity. Using atomic layer deposition in a fluidized bed reactor, we prepared TiO2 (P25) samples with Pt loadings ranging from 0.04 wt % to around 3 wt %. In order to reveal the mechanism behind the photocatalytic behavior of Pt on P25, we investigated the different aspects (i.e., surface area, reactant adsorption, light absorption, charge transfer, and reaction pathway) of heterogeneous photocatalysis individually. In contrast to the often proposed prolonged lifetime of charge carriers in Pt-loaded TiO2, we found that after collecting the excited electrons, Pt acts more as a recombination center independent of the amount of Pt deposited. Only when dissolved O2 is present in the solution, charge recombination is suppressed by the subsequential consumption of electrons at the surface of the Pt clusters with the dissolved O2 benefited by the improved O2 adsorption on the Pt surface.