Transport properties of silica membranes derived from the oxidative thermolysis of crosslinked polydimethylsiloxane (PDMS) were studied between 35-100 degrees C. Changes in pore morphology and transport properties were investigated for varying thermolysis and crosslinking treatments. Permeation and pressure-decay sorption measurements were made for CO2, H-2, He, O-2, N-2, and CH4. Physisorption data for CO2 and N-2 were also analyzed to characterize the pore size distributions of these materials. The activation energies of permeation and diffusion were measured along with the enthalpies of sorption to compare the effects of varying fabrication methods. All samples showed similar enthalpies of sorption. Regular and extended-thermolysis heat treated silica showed similar permeabilities and diffusion coefficients, but the heat treated silicas showed an average increase of 2.3 kJ/mol in their activation energies of permeation. The permeation, sorption, and diffusion selectivities of various gas pairs were compared. The silicas showed high sorption selectivity in gas pairs with large differences in critical temperature, but showed low diffusion selectivity due to low entropic selectivity. Overall, the PDMS-derived silicas show promise, but require further tuning to improve the entropic selectivity of penetrants.