Highly branched alkoxysilane (HB) units were prepared in situ via a Michael-type reaction between pentaerythrithol triacrylate and aminopropyltriethoxysilane. These units were used as an inorganic component for the modification of cellulose acetate (CA) films using the sol-gel process. The thermal and dynamic-mechanical behaviors, the morphology, and the dimensional stability of the modified CA films were analyzed. The siloxane-modified CA films showed thermal stability similar to pure CA, but the residue content at 900 degrees C increased with the addition of HB units. The morphology of these films was characterized by siloxane nanodomains dispersed in the CA matrix, with good interfacial adhesion between the phases. Moreover, the CA/siloxane nanocomposite films showed improved dimensional stability in comparison with CA, i.e., in the presence of HB, the dimensional change was reduced to around 50% of the value observed for pure CA. Finally, a complex dynamic-mechanical behavior was obtained for the nanocomposite films, as a consequence of the heterogeneous morphology. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 2559-2566, 2011