Silylation of cellulose acetate having different acetyl contents (1: DSAc = 1.80 and 2: DSAc = 2.46) was carried out by the reaction of various chlorosilanes [R = SiMe3 (a), SiEt3 (b), SiMe2C8H17 (c), SiMe2C18H37 (d), SiMe2Ph (e)] with remaining hydroxy groups of cellulose acetate, and complete substitution was confirmed by H-1 NMR and FT-IR spectroscopy. All of the silylated derivatives (la-e and 2a-e) were soluble in common organic solvents and displayed enhanced solubility in relatively non-polar solvents. The onset temperatures of weight loss of la-e and 2a-e were higher than 220 degrees C, indicating fair thermal stability. Silylation of I and 2 accompanied a decrease in glass transition temperature, and the decrease became more pronounced as the length of alkyl chain in the silyl group increased. Free standing membranes of la-e and 2a-e could be fabricated, and the silylated derivatives displayed remarkably improved gas permeability as compared to 1 and 2. Incorporation of longer silyl groups was more effective to enhance gas permeability, for instance, the P values of 1c were the highest, and its P-CO2 (160 barters) was 94 times higher than that of 1. The P-CO2/P-N2 of the silylated polymers (1a-e and 2a-e) was appreciably large (19-36), and the values for 1a-c, 2a, and 2c were located either on the upper bound or above it in the plot of permselectivity versus permeability for the CO2/N-2 gas pair. (c) 2007 Elsevier B.V. All rights reserved.