A silicon oxide (SiOx) layer of molecular-scale thickness, which we have named an "oxide nanoskin", has been successfully formed on a poly(methyl methacrylate) (PMMA) substrate through the vapor-phase chemisorption of organosilane and subsequent photooxidation using 172-nm vacuum ultraviolet (VUV) light. First, a hydrophilized PMMA sample was exposed to the vapor of a precursor, that is, tetraethoxysilane (TEOS). As was confirmed by water contact-angle measurements and X-ray photoelectron spectroscopy (XPS), chemisorption of the TEOS molecules was accomplished after 3 h of vapor treatment. Next, VUV irradiation was conducted for more than 10 min, and the chemisorbed TEOS molecules were photochemically converted to SiOx, as was evidenced by XPS. The resulting oxide layer was confirmed by transmission electron microscopy to be continuous with a thickness of 3 nm or less. As a control experiment, a SiOx layer was deposited by the same procedure on a Si substrate covered with a native oxide. As was determined by ellipsometry, the thickness of the oxide deposited on the substrate was found to be only about 0.45 nm, which was much smaller than that of the SiOx layer fabricated on the PMMA substrate. As a result of the formation of this oxide nanoskin layer, the charge density and surface acidity of the PMMA sample became almost equal to those of a native oxide-covered Si substrate. The PMMA surface covered with the oxide nanoskin layer showed excellent hydrophilicity even after being aged in air for 30 days.