Gas permeability is one of the key performance characteristics of polymeric films. Here we explore a novel approach for modifying the oxygen transmission rate of polymers by vapor deposition of melamine. We win show that melamine, upon physical vapor deposition, forms a transparent uniform layer on polymeric films such as biaxially oriented polypropylene. X-ray diffraction, in combination with molecular modeling, has indicated that the melamine coating is crystalline and macroscopically oriented with the preferred direction of the melamine molecules parallel to the (polymeric) substrate. The vapor-deposited melamine molecules undergo a large number of cooperative hydrogen bond interactions leading to a coherent layer of an infinite supramolecular network. The melamine coating acts as a surprisingly effective barrier against gases, decreasing drastically, for example, the oxygen transmission rate through coated polymer films by 2 orders of magnitude. The effect is attributed to the crystalline nature of the deposited melamine layer, strengthened by the high level of hydrogen bonding. This is the first example of application of supramolecular chemistry for the production of health and environment friendly transparent barrier coatings against oxygen. The vacuum-coating process with melamine and related compounds is expected to bring a major breakthrough in the field of transparent polymeric barrier films for applications, for example, in food and pharmaceutical packaging.