Polydimethylsiloxane (PDMS) can be deposited on various substrates using chemical vapor deposition process, which results in the formation of PDMS thin films with thickness below 5 nm. PDMS layers can be evenly deposited on surfaces of nanoparticles composed of various chemical compositions such as SiO2, TiO2, ZnO, C, Ni, and NiO, and the PDMS-coated surface becomes completely hydrophobic. These hydrophobic layers are highly resistant towards degradation under acidic and basic environments and UV-exposures. Nanoparticles coated with PDMS can be used in various environmental applications: hydrophobic silica nanoparticles can selectively interact with oil from oil/water mixture, suppressing fast diffusion of spill-oil on water and allowing more facile physical separation of spill-oil from the water. Upon heat-treatments of PDMS-coated TiO2 under vacuum conditions, TiO2 surface becomes completely hydrophilic, accompanying formation oxygen vacancies responsible for visible-light absorption. The post-annealed PDMS-TiO2 shows enhanced photocatalytic activity with respect to the bare TiO2 for decomposition of organic dyes in water under visible light illumination. We show that the simple PDMS-coating process presented here can be useful in a variety of field of environmental science and technology.