The pure-gas sorption, diffusion, and permeation properties of ethylbenzene in poly(dimethylsiloxane) (PDMS) are reported at 35, 45, and 55 degrees C and at pressures ranging from 0 to 4.4 cmHg. Additionally, mixed-gas ethylbenzene/N-2 permeability properties at 35 degrees C, a total feed pressure of 10 atm, and a permeate pressure of 1 atm are reported. Ethylbenzene solubility increases with increasing penetrant relative pressure and can be described by the Flory-Rehner model with an interaction parameter of 0.24 +/- 0.02. At a fixed relative pressure, ethylbenzene solubility decreases with increasing temperature, and the enthalpy of sorption is -41.4 +/- 0.3 kJ/mol, which is independent of ethylbenzene concentration and essentially equal to the enthalpy of condensation of pure ethylbenzene. Ethylbenzene diffusion coefficients decrease with increasing concentration at 35 degrees C. The activation energy of ethylbenzene diffusion in PDMS at infinite dilution is 49 +/- 6 kJ/mol. The ethylbenzene activation energies of permeation decrease from near 0 to -34 +/- 7 kJ/mol as concentration increases, whereas the activation energy of permeation for pure N-2 is 8 +/- 2 kJ/mol. At 35 degrees C, ethylbenzene and N-2 permeability coefficients determined from pure-gas permeation experiments are similar to those obtained from mixed-gas permeation experiments, and ethylbenzene/N-2, selectivity values as high as 800 were observed.