The oxidation of aniline with ammonium peroxydisulfate in water without added acid was studied in dilute reaction media. The new nanostructured nonconducting polyaniline sulfate/hydrogen sulfate, synthesized by dilute dopant-free template-free falling-pH method, consists of network of nanorods and nanogranules. The molecular structure of polyaniline, investigated by the elemental analysis, FTIR, Raman, UV-Vis and EPR spectroscopies, was found to be significantly different in comparison with the structure of the nanostructured PANI sulfate/hydrogen sulfate prepared by dopant-free template-free falling-pH method in concentrated aqueous aniline solutions. The mechanism of the formation of nanostructured polyaniline under dopant-free template-free dilute reaction conditions is discussed. (C) 2011 Elsevier B.V. All rights reserved.