Nanofibrous PANI was synthesized by the oxidative polymerization of aniline, without the addition of acid, in the presence of oxidants (mixture of ammonium peroxydisulfate and H2O2) in excess. The reaction course was monitored by measuring temperature and acidity changes over time. The influence of reaction time on the structure, morphology and electrical conductivity of the obtained products was examined. Evolution of the molecular structure of the reaction intermediates/products was studied by in situ Raman spectroscopy. The molecular structure of isolated, powdered products was studied by FTIR spectroscopy, and their morphology was characterized by scanning electron microscopy. It has been found that electrical conductivity of products increases with the reaction time due to the increase in amount of the conductive polaron form of PANI. Longer and better defined nanofibers, with higher electrical conductivity are present in PANI isolated during the second exothermic phase in comparison with the one isolated at the beginning of this phase.