Polymer nanocomposites consisted of biodegradable poly(3-hydroxybutyrate) (PHB) and organically modified montmorillonite Cloisite30B (30B) have been prepared by the solution casting method and isothermally degraded at 230, 235, 240 and 245 degrees C. The addition of 30B increases the thermal stability of PHB and the most pronounced effect has the addition of 1 wt.% of 30B. Kinetic analysis was performed using model-free isoconversional and reduced time plots (RTP) methods. The isothermal degradation of pure PHB and PHB/30B nanocomposites can be separated in two distinct regions: the first where small mass loss appears and the second. assigned as the main degradation mechanism, where the main mass loss takes place. The empirical kinetic triplets (E, A. and g(a)) for the main degradation mechanism were determined. (C) 2008 Elsevier B.V. All rights reserved.