Polyaniline nanomaterials have gained significant interest because of their unique electronic properties, simple synthesis process and their environmental stability. The morphology of polyaniline has been widely studied due to both its strong influence on the properties and its various applications. We report on the cerium(IV) ammonium nitrate mediated synthesis of polyaniline nanospheres using an interfacial polymerization technique in which polyaniline serves as a guest of the cerium(III) ion, a paramagnetic species produced during the synthesis condition. Cerium(III) ionic species bonded with the chain nitrogen of the polyaniline and the supramolecular system show the paramagnetic behavior throughout the experimental temperature range 400-1.9 K. (C) 2010 Elsevier B. V. All rights reserved.