A strong effect of the electrospinning conditions on NiO microstructure was noticed. Viscous solutions containing nickel nitrate (Ni(NO3)(2)), polyvinylpyrrolidone (PVP), ethanol (C2H5OH), and H2O were prepared. Produced electrospun fibers were thermally treated. The chemical composition of the viscous solution, the time of electrospinning and calcination played a crucial role in the formation of corresponding nickel oxide (NiO) microstructures. At a lower Ni(NO3)(2) concentration a porous microstructure consisting of interconnected fibers was obtained, whereas at a higher Ni(NO3)(2) concentration a laminar microstructure was obtained. The formation of metallic Ni was assigned to the reducing action of PVP during the heating of electrospun fibers. Based on EDS and XRD analyses it was suggested that metallic Ni was restricted to the surface of NiO particles. With a prolonged heating time at increased temperature metallic Ni was reoxidized to NiO. (C) 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.