Cellulose acetate (CA) remains one of the few polymers to be commercialized for use in industrial CO2/CH4 gas separations, despite the fact that other polymeric materials, such as polymers of intrinsic microporosity and thermally rearranged polymers, have demonstrated higher gas separation performance. The availability of this renewable polymer, in addition to its high inherent selectivities and facile processing, make CA a preferred candidate for industrial applications. This study describes a modification for fabricating improved CA membranes through acid-catalyzed silanation of dense CA films. Processing advantages include using neat CA films and a reaction temperature well below the glass transition temperature of CA. In addition, the resulting modified CA membranes exhibit substantially higher gas permeabilities for both pure and mixed gas feeds. In comparing neat CA to modified CA, CO2 permeability increases from 4.6 to 24.5 Barrer in 100 psi pure gas feeds and from 2.7 to 12.7 Barrer in 800 psi mixed gas feeds. The crosslinking present in the modified CA also increases plasticization resistance in high pressure mixed gas streams, which causes modified CA's CO2/CH4 selectivity to increase from 25.3 at 200 psi to 26.1 at 800 psi, whereas neat CA's CO2/CH4 selectivity markedly drops from 35.9 at 200 psi to 28.8 at 800 psi. TGA and gel fractions, as well as FTIR NMR and SEM/EDS, confirm that this acid-catalyzed silanation reaction both silanates and crosslinks the membrane.