Silicone rubber is universally regarded as the best-in-class elastomer for extreme environments. In addition, silicone rubber is one of the most permeable elastomers. This property is a key advantage for silicone rubber in many design situations. However, some applications which require silicone performance in extreme environments also require low gas permeability. Applications that require such material characteristics cover a range of aerospace components such as inflatables, ducting, and diaphragms. This paper presents a technical review of the gas permeability characteristics of silicone rubber, including analysis of the gas penetration mechanism through the elastomer. The chemical structure of silicone rubber affects permeability, as do other governing factors such as temperature and gas type. In particular, phenyl vinyl methyl silicone (PVMQ) has the lowest glass transition temperature (-120°C) of all elastomers, but its gas permeability is very high. Arlon has developed a proprietary technology to reduce the gas permeability of PVMQ. Arlon's technological approach produces large gains in gas permeability reduction without sacrificing excellent low temperature elastomeric flexibility.