Composites based on cellulose acetate/niobium and chitosan/niobium have been obtained by a new synthetic hydrothermal route with different amounts of Nb2O5 (0.0%; 3.7%; 5.8%; 6.7% and 10.9%, incorporated on cellulose acetate fibers and 0.0%; 3.0%; 9.3% and 13.9%, attached on chitosan surface). Thermodecomposition studies were carried on 30-800 degrees C range at rate of 10 degrees C min(-1). Kinetic parameters like activation energy, pre-exponential factor, Gibbs energy, enthalpy and entropy of activation were determined by using Coats-Redfern equation. The activation energy (E) of degradation presented a decreasing with the incorporation of niobium on cellulose acetate, whereas all kinetic parameters maintained constant with the incorporation of niobium, for chitosan what resulted in a more stable that hybrid chitosan/Nb2O5 composites. (c) 2007 Elsevier Ltd. All rights reserved.