Abstract The effect of organic substituents and polymer backbone structure on the oxygen and nitrogen permeability of the following silicone polymers was investigated: Series A, (MeRSiO) x, where R is alkyl, aryl, or CH 2CH 2CF 3; Series B, (Me 2SiC nH 2nSiMe 2O) x, where n=2, 6 and 8; Series C, (Me 2SiC 6H 4SiMe 2O) x. It was found that as the size of the alkyl group in Series A was increased, the permeability decreased markedly, whereas the O 2-N 2 selectively remained more or less constant. Aryl substituents in Series A and p-silarylene linkages in Series C caused a significant decrease in permeability but an increase in the selectivity. Incorporation of organosilyl pendant groups onto organic polymers causes pronounced increases in permeability but decreases in selectivity. The cost for the oxygen enrichment of air using these silicone membranes was analyzed using computer modelling. A quantitative permeability-selectivity-cost relationship has been established.