We report on the formation of polymeric thin films on a solid surface using gaseous monomers. The process consists of adding catalysts on the surface of a solid substrate and polymerizing volatile monomers at the gas/solid interface, or directly using the vapors of the catalyst and the monomers to selectively form polymers inside the microchannels. The morphology and properties of these thin films depend on the concentration of catalysts, the time of exposure to the monomers, and the type of the monomers, which can be easily controlled at the molecular level. This solventless process can be used for a range of applications.