The goal of this work is to analyze the effect of carbon nanotubes (CNTs) on the pyrolysis of either high density polyethylene (PE) or polypropylene (PP) matrices by using both kinetic thermogravimetric analyses (TGA) under non-isothermal conditions and a fixed-bed reactor under isothermal conditions. Under non-isothermal conditions, CNTs increased the beginning of thermodegradation for both matrices with differences as high as 30 degrees C and 22 degrees C as compared with neat PP and PE, respectively. This enhanced thermal stability in PP based composites was associated with an increase in the apparent activation energy whereas in PE based composites lower pre-exponential factors associated with reduced conformational entropy, are responsible for the enhanced thermal stability. The thermodecomposition processes were studied by assuming geometrical contraction and nucleation models. The invariant pre-exponential factor and apparent activation energy obtained were quantified for each sample confirming that these values depended on the polymer matrix and concentration of CNTs. These invariant parameters were in good agreement with those obtained by isoconversional analyses allowing the prediction of the thermogravimetric behavior. Our findings clearly showed the strong effect of CNTs on the non-isothermal pyrolysis of polymer materials changing its kinetic and the activation energy. Results from isothermal pyrolysis (450 degrees C-40') confirmed the thermal stability by the presence of CNTs as higher condensable (C-9-C-40) and lower gas (C-1-C-4) yields in PP-CNTs composites, and a higher amount of unreacted polymer and a lower both condensable and gas yields for PE-CNT, as compared with the pure matrix, were found. (C) 2015 Elsevier Ltd. All rights reserved.