Surface modification methods based on a combination of plasma and chemical treatments render an unfilled butyl rubber surface highly reactive toward organosilanes, allowing formation of an organosilane self-assembled monolayer (SAM). Plasma oxidation of the butyl rubber surface followed by vapor deposition of SiCl4 produces a hydrophilic surface suitable for anchoring organosilanes. Fabrication of SAMs on this hydrophilic butyl rubber surface with n-octadecyltrichlorosilane (OTS) and trichloro(1H,1H,2H,2H-perfluorooctyl)silane (FOTS) via vapor deposition resulted in a 15% and 25% decrease in gas permeability, respectively, with no change in optical transparency of the butyl rubber.