The solubility, diffusivity, and permeability of ethylbenzene in poly(1trimethylsilyl-1-propyne) (PTMSP) at 35, 45 and 55 ยฐC were determined using kinetic gravimetric sorption and pure gas permeation methods. Ethylbenzene solubility in PTMSP was well described by the generalized dual-mode model with ฯญ 0.39 ฯฎ 0.02, b ฯญ 15 ฯฎ 1, and Cะ H ฯญ 45 ฯฎ 4 cm 3 (STP)/cm 3 PTMSP at 35 ยฐC. Ethylbenzene solubility increased with decreasing temperature; the enthalpy of sorption at infinite dilution was ฯช40 ฯฎ 7 kJ/mol and was essentially equal to the enthalpy change upon condensation of pure ethylbenzene. The diffusion coefficient of ethylbenzene in PTMSP decreased with increasing concentration and decreasing temperature. Activation energies of diffusion were very low at infinite dilution and increased with increasing concentration to a maximum value of 50 ฯฎ 10 kJ/mol at the highest concentration explored. PTMSP permeability to ethylbenzene decreased with increasing concentration. The permeability estimated from solubility and diffusivity data obtained by kinetic gravimetric sorption was in good agreement with permeability determined from direct permeation experiments. Permeability after exposure to a high ethylbenzene partial pressure was significantly higher than that observed before the sample was exposed to a higher partial pressure of ethylbenzene. Nitrogen permeability coefficients were also determined from pure gas experiments. Nitrogen and ethylbenzene permeability coefficients increased with decreasing temperature, and infinite dilution activation energies of permeation for N 2 and ethylbenzene were ฯช5.5 ฯฎ 0.5 kJ/mol and ฯช74 ฯฎ 11 kJ/mol, respectively.