Different composites were formed incorporating three different polyhedral oligomeric silsesquioxane (POSS) molecules into polyoxymethylene (POM) by melt blending. The melting temperature (Tm) and crystallinity degree (xc) of POM remained practically constant for two of the composites. Tm slightly decreased and xc slightly increased for the other system. Whatever the POSS used, the thermal stability of POM remarkably increased, leading to a maximum of approximately 50 ºC with one of the POSS molecules.