Mean permeability coefficients, PBAR, for He, O2, N2, CH4, and CO2 in ethyl cellulose with ethoxy contents of 47.2, 47.9, and 49.6% were determined at 35.0-degrees-C and at pressures from 4 to 13.6 atm. (405.2 to 1378.0 kPa). The values of PBAR increase with increasing ethoxy content because of a decrease in the chain packing density. By contrast, the overall selectivity of ethyl cellulose to several gas pairs is relatively unchanged by variations in the ethoxy content of the polymer. The glass-transition temperature of ethyl cellulose, and hence the chain mobility, is also little affected by the ethoxy content. Ethyl cellulose exhibits a considerably higher gas permeability and a lower selectivity than the structurally similar cellulose acetate with a comparable degree of substitution. This appears to be due to the larger free volume and lower glass-transition temperature of the former polymer.