Highly crosslinked poly(dimethylsiloxane) (PDMS) is discussed as an alternative membrane material for the gas separation of highly concentrated chlorine gas (90-95 vol %) and oxygen, due to an initial high permeation for Cl-2 and a high selectivity of Cl-2/O-2. It was found that the separation properties of the PDMS membrane change over time upon exposure to aggressive chlorine gas; the flux will go down, and the material may even degrade if not appropriately prepared and protected. The PDMS was exposed to chlorine gas over 4 weeks in a glass chamber at both 30 and 60degreesC and analysed by (FTIR). The membranes were exposed to chlorine gas in a permeation cell with measurements of the permeability of N-2, O-2, and Cl-2 at regular intervals. The temperature range for the permeation measurements was 30-100degreesC, and the pressure difference over the membrane was ca. 2 bar. The time of exposure in the permeation cell was several weeks. The absorption of N-2, O-2, and Cl-2 in the PDMS at temperatures in the same range was also measured. This article discusses the durability of the highly crosslinked PDMS membrane following chlorine exposure.