Dimeric and oligomeric At and Zn-containing isobutyl silsesquioxanes (POSS) were studied to investigate their thermal stability both in inert and oxidant atmosphere, by means of Differential Scanning Calorimetry (DSC) and thermogravimetric analyses (TGA). Thermo-oxidative degradation leads to an important solid residue yield, as a consequence of the POSS oxidation to a thermally stable ceramic phase. The study of the differences in degradation pathways of both metal POSS derivatives clearly shows a competition between two possible mechanisms for product thermal behaviour, namely evaporation and oxidation. The metal POSS compounds were incorporated via melt blending into a PP matrix and the composites thermal properties were studied by thermogravimetric analyses. Thermo-oxidative degradation of composites is strongly affected by the presence of metal containing POSS, resulting in an improved thermal stability, in terms of higher weight loss temperature. (c) 2005 Elsevier Ltd. All rights reserved.