In this paper, the influence of the functionalisation of polyhedral oligomeric silsesquioxanes (POSS) cages on the preparation and properties of polypropylene (PP) based nanocomposites is studied. POSS with different chain lengths (octamethyl-, octaisobutyl- and octaisooctyl-POSS) are taken in to account and melt mixed with PP. The resulting composites are characterised as regards thermal and morphological characteristics by means of DSC, TGA, SEM and X-ray diffraction. A good dispersion is obtained particularly at low loadings of POSS functionalised with longer organic chains. Important features are recognised as regards the crystallisation behaviour of PP by octamethyl- and octaisobutyl-POSS: The former acting as a nucleating agent and the second inducing PP polymorphism. (c) 2005 Elsevier Ltd. All rights reserved.