A modified co-axial electrospinning process using salt solutions as sheath fluids for preparing polymer nanofibers was investigated. A series of polyvinylpyrrolidone (PVP) fibers were prepared with NaCl aqueous solutions at varying concentrations as sheath fluids. The sheath fluid had a significant influence on the formation of the compound Taylor cone. Scanning electron microscopy results demonstrated that the diameters of PVP nanofibers could be manipulated through the concentration of NaCl solutions within an appropriate range. With 2 mg ml(-1) NaCl solution as sheath fluid, the smallest PVP nanofibers, with a diameter of 120 +/- 40 nm, were obtained. Co-axial electrospinning with salt solutions as sheath fluids is a facile method for achieving finer, homogeneous polymer nanofibers. (C) 2011 Elsevier B.V. All rights reserved.