The permeabilities and selectivities of O 3 , O 2 , and N 2 through silicone capillary membranes employed to degrade organic pollutants in water or air have been experimentally determined. These characteristics have been studied for silicone membranes used in membrane reactors having the following conditions: O 3 in O 2 on one side of the membrane, and either water containing pollutants or a perfluorocarbon (FC) phase containing pollutants on the other side. The permeability of O 3 (8.8 e-13 kgmolrm/m 2 rsrkPa) is four times that of O 2 through virgin silicone rubber. Exposure to O 3 modifies the polymer and alters the permeabilities of O 3 and O 2 . The presence of water with O 3 leads to an increase in O 3 and O 2 permeability ( ร 30%) and an increase in the selectivity, a O 2 0N 2 (ร 10%). The increased permeabilities are likely to be due to the formation of peroxides on the surface and possibly in the polymer. When the silicone capillary membranes were exposed to a perfluorocarbon (FC), the permeabilities of O 3 and O 2 decreased ( ร 9%) due to an increase in crosslinking in the polymer matrix; there was also a slight increase in a O 2 0N 2 (ร 2%), which can be ascribed to the smaller molecular sieving radius of O 2 compared to N 2 .