Perfluoroacylated derivatives of ethyl cellulose [R = CF3CO (2a), C2F5CO (2b), C3F7CO (2c), C7F15CO (2d), C6F5CO (2e)] were synthesized in good yields by the reaction of various perfluoroacylating agents with residual hydroxy groups of ethyl cellulose (1; DSEt, 2.69). FTIR spectra of the resulting polymers 2a-d furnished the evidence for complete substitution of hydroxy protons by the perfluoroacyl groups. All the derivatives (2a-e) were soluble in common organic solvents and displayed enhanced solubility in moderately polar aprotic and nonpolar solvents. The onset temperatures of weight loss of 2a-d in air were higher than 270 C, indicating fair thermal stability. Free-standing membranes of 1 and 2a-e were fabricated, and 2a-d exhibited large contact angle with water and enhanced gas permeability (P) as compared to 1. An optimum increment in the size of the perfluoroacyl group led to the largest increment in the gas permeability of the polymers; i.e., 2c exhibited the highest P values (e.g., P-CO2 284 barrers; cf. P-CO2 of 1 110 barrers). The PCO2/PN2 and PCO2/PCH4 permselectivity values of the polymers (2a-e) were in the range of 14-22 and 8-11, respectively.