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Tungsten oxide (WO 3 ) nanofibers with shapes ranging from cylindrical to ribbon-like were prepared by annealing electrospun polyvinylpyrrolidone/ammonium metatungstate (PVP/AMT) fibers. Formation of periodically folded "zigzag" patterns in PVP/AMT and WO 3 ribbon-like fibers was observed for the first time at the initial stage of the electrospinning process on the surface of a stationary substrate. Among methods tested (capillary needle, needleless dc-and ac-electrospinning), only the capillary needle dc-electrospinning process was effective in producing ribbon-like fiber structures. Annealing of such PVP/AMT fibers at 500 C in air led to the formation of 80 AE 10 nm thick WO 3 ribbons with a width-tothickness ratio of up to 50 : 1. Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), X-ray diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), and Raman spectroscopy were used to analyze the material. Analyses revealed that regardless the fiber's shape, the annealed oxide fibers were polycrystalline with a grain size of 60 AE 30 nm and consisted of the monoclinic phase of WO 3 . When compared to cylindrical fibers, the ribbon-like WO 3 nanofibers exhibited higher porosity but lower mechanical strength with increased width of the ribbon-like structure. Fig. 7 XPS (W4f, C1s, O1s, and N1s) spectra of (a) as prepared PVP/AMT ribbon-like nanofibers and (b) the resulting WO 3 fibers after thermal processing for 3 h at 500 C in the air; (c) FTIR spectra of PVP/AMT (top) and annealed WO 3 (bottom) fibers.This journal is