Pure gas solubility and permeability of H 2 , O 2 , N 2 , CO 2 , CH 4 , C 2 H 6 , C 3 H 8 , CF 4 , C 2 F 6 , and C 3 F 8 in poly(1-trimethylsilyl-1-propyne) (PTMSP) were determined as a function of pressure at 35Β°C. Permeability coefficients of the perfluorinated penetrants are approximately an order of magnitude lower than those of their hydrocarbon analogs, and lower even than those of the permanent gases. In striking contrast to hydrocarbon penetrants, PTMSP permeability to fluorocarbon penetrants decreases with increasing penetrant size. This unusual size-sieving behavior in PTMSP is attributed to low perfluorocarbon solubilities in PTMSP coupled with low diffusion coefficients relative to those of their hydrocarbon analogs. In general, perfluorocarbon penetrants are less soluble than their hydrocarbon analogs in PTMSP. The difference in hydrocarbon and perfluorocarbon solubilities in high free volume, hydrocarbon-rich PTMSP is much smaller than in hydrocarbon liquids and liquidlike polydimethylsiloxane. The low solubility of perfluorocarbon penetrants is ascribed to the large size of the fluorocarbons, which inhibits their dissolution into the densified regions of the polymer matrix and reduces the number of penetrant molecules that can be accommodated in Langmuir sites. From the permeability and sorption data, diffusion coefficients were calculated as a function of penetrant concentration. With the exception of H 2 and the C 3 analogs, all of the penetrants exhibit a maximum in their concentration-dependent diffusion coefficients. Resolution of diffusion coefficients into a mobility factor and a thermodynamic factor reveals that it is the interplay between these two terms that causes the maxima. The mobility of the smaller penetrants (H 2 , O 2 , N 2 , CH 4 , and CO 2 ) decreases monotonically with increasing penetrant concentration, suggesting that the net free volume of the polymer-penetrant mixture decreases as additional penetrant is added to PTMSP. For larger penetrants mobility either: (1) remains constant at low concentrations and then decreases at higher penetrant concentrations (C 2 H 6 , CF 4 , and C 2 F 6 ); (2) remains constant for all concentrations examined (C 3 H 8 ); or (3) increases monotonically with increasing penetrant concentration (C 3 F 8 ). Presumably these results reflect the varying effects of these penetrants on the net free volume of the polymer-penetrant system.