During the preparation of Fe3O4/PVC composite nanofibers by electrospinning of FeCl3 solutions in PVC/THF/DMF system was observed a strong dependence of the formation mechanism of Fe3O4 nanoparticles on FeCl3 concentration, with an optimum at 2.91% FeCl3 in PVC. To explain the existence of this optimum is proposed for the first time an original mechanism, well supported by the results of structural analysis of the composite nanofibers by XRD, FTIR and SEM. (C) 2010 Elsevier B.V. All rights reserved.