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The effect of poly[styrene-b-(ethylene-co-butylene)-b-styrene] (SEBS) copolymer on the thermal and dielectric properties of polypropylene (PP)—nanosilica (NS) composites in relation with morphological aspects revealed by atomic force microscopy (AFM) was investigated in this article. SEBS hindered the crystallization process of PP in PP/NS composites, leading to a smaller degree of crystallinity and lower perfection of crystalline structure. Broader lamellar thickness distribution was obtained in nanocomposites containing SEBS. Almost two times higher dielectric loss as compared to PP reference and two relaxation processes were detected in εr ′′(f) curves of nanocomposites. The first peak, in the same frequency domain as for the references, was assigned to α-relaxation of polymer components together with interfacial polarization. The relaxation time follows the Arrhenius law with an activation energy of 80–90 kJ/mol. For the second process, the temperature dependence of the relaxation times obeyed the VFT equation. The dielectric changes following the incorporation of SEBS support its tendency to hinder the motional processes in PP, in accordance with DSC results. A smooth transition from a phase rich in SEBS to one containing mainly PP was detected in the AFM image of the composite with the larger amount of SEBS, emphasizing the good compatibility at the PP/SEBS interface. POLYM. ENG. SCI., 53:2081–2092, 2013. © 2013 Society of Plastics Engineers