The thermal degradation behaviour of poly (L-lactide) nanocomposites containing both as-received (MWCNT) and purified multi wall carbon nanotubes (p-MWCNT) was evaluated by means of thermogravimetric analysis (TGA). Composites with carbon nanotube contents of 0, 0.75, 1.25, 2.5, 4 and 5 wt. % were prepared. The thermal degradation process was analyzed in the light of Kissinger and Ozawa-Flynn-Wall (OFW) models revealing a lowering of the activation energy (E) due to iron and aluminium residues present in carbon nanotubes.