Nanocomposite materials were obtained by blending multi-wall carbon nanotubes (CN), obtained by acetylene catalytic chemical vapour deposition (CVD) on Co/Fe-modified NaY zeolite, with syndiotactic poly(propylene) (sPP). The nanotubes, well dispersed in the polymer matrix, favour the crystallization of the sPP helical chains and significantly improve the sPP thermal stability either in nitrogen or in air. The morphology of the sPP affects the behaviour of the sPP degradation in air.