Mean permeability coefficients for CH4 and CO2 ((P) over bar(CH4) and (P) over bar(CO2)) in cellulose acetate (CA, DS = 2.45) were determined at 35 degrees C (95 degrees F) and at pressures up to about 54 atm (800 psia). The measurements were made with pure CH4 and CO2 as well as with CH4/CO2 mixtures containing 9.7, 24.0, and 46.1 mol % CO2. In the measurements with the pure gases, (P) over bar(CH4) was found to decrease with increasing pressure, as expected from the "dual-mode" sorption model. By contrast, (P) over bar(CO2), passes through a minimum and then increases with increasing pressure, probably due to the plasticization (swelling) of CA by CO2. The values of (P) over bar(CH4), and (P) over bar(CO2) determined with the mixtures containing 9.7 and 24.0 mol % CO2 decrease with increasing total pressure; this behavior is adequately described by the extended "dual-mode" sorption model for mixtures. By contrast, the values of (P) over bar(CH4) and (P) over bar(CO2), obtained with the mixture containing 46.1 mol % CO2 pass through a minimum and then increase as the total pressure is raised, probably also due to the plasticization of CA by CO2. The CO2/CH4 selectivity (=(P) over bar(CO2)/(P) over bar(CH4)) of the CA membranes decreases with increasing total pressure and, at constant pressure, decreases with increasing CO2 concentration in the feed mixture. The effects of exposing the CA membranes to high-pressure CO2 prior to the permeability measurements ("conditioning" effects) on (P) over bar(CH4) and (P) over bar(CO2) have also been studied.