Cellulose acetate oleate ester has been synthesized from oleic acid and cellulose acetate by using a mixed anhydride system based on a mixture of trifluoroacetic acid and trifluoroacetic acid anhydride with chloroform as a co-solvent. In absence of oleic acid, cellulose acetate was deacetylated in this mixed anhydride system. However, when oleic acid was added, hydrolysis was prevented and oleate groups bound to the cellulose acetate chain were detected as confirmed by Nuclear Magnetic Resonance spectroscopy. Compared to cellulose acetate, cellulose acetate oleate ester showed a mechanical behavior closer to ductile materials and a lower glass transition temperature, indicating that oleate groups can act as an internal plasticizer of the polymer chains. The contact angle values increased from 61 degrees for cellulose acetate to 106 degrees for cellulose acetate oleate, which presented a low surface energy value (16 mJ/m(2)) with no contribution of the polar component. Furthermore, the esterified cellulose acetate oleate material showed a decrease in the water vapor transmission rates by similar to 76% compared to those of pure cellulose acetate films. The oxygen permeability was also decreased by similar to 90% after grafting the oleate group onto the cellulose acetate repetitive units because of the creation of a densely packed matrix, confirmed by SEM analysis. This behaviour was attributed to the oleate chain length, which creates a chemical interaction between water and oxygen molecules with free hydroxyl and oleate groups of cellulose acetate oleate.