A mixed ester of ethyl cellulose (EC) has been prepared by reaction of trifluoroacetic anhydride with the residual hydroxy groups of ethyl cellulose. The mixed eater is soluble in tetrahydrofuran, dichloromethane, chloroform, benzene and pyridine. FTIR and NMR spectra show that hydroxy groups of ethyl cellulose were replaced by trifluoroacetoxy groups. The trifluoroacetyl ethyl cellulose (TFAEC) has higher selectivity for oxygen relative to nitrogen, in gas transport, than unmodified EC. Annealing at an elevated temperature further improves selectivity for oxygen, whilst subsequent ageing at ambient temperature partially reduces oxygen selectivity. The tensile strength of TFAEC is virtually the same as that of unmodified EC, but the elongation to break is 200% higher than for EC.