We applied hydrophilic surface modification to P-25 TiO2 nanoparticles via thermal deposition of polydimethylsiloxane (PDMS), followed by annealing at 800 degrees C under vacuum conditions. This process yielded a thin layer consisting of hydrophilic organic functional groups (e.g., -C = O) as well as SiOx structures on the surface of TiO2. Compared to bare TiO2, the surface-modified TiO2 showed a higher adsorption capacity and photo-catalytic decomposition rate toward methylene blue. However, regarding the photo-catalytic decomposition of phenol, total mineralization of phenol was more complete when bare TiO2 was used as a photo-catalyst, whereas partial oxidation of phenol (i.e., into hydroquinone) was more dominant and total oxidation of phenol was supressed in the presence of surface-modified TiO2. Overall, one cannot simply say that hydrophilic modification of TiO2 leads to a higher affinity to H2O molecules with a higher yield of strongly-oxidizing agents (OH radicals) to increase the photo-catalytic activity in aqueous solutions. Our results imply that a subtle balance of the adsorption energy or rates of H2O, reactants, and reaction intermediates can be important factors for determining the photo-catalytic reaction rate. Different types of photo-catalyst surface modification can be beneficial or detrimental depending on the reaction. It is highlighted that one should be wary of evaluating the overall photocatalytic activities of dissimilar catalysts based on the results of only a couple of reactions. (C) 2017 Elsevier B.V. All rights reserved.