© 2015 Elsevier Ltd and Techna Group S.r.l. Tungsten oxide (WO3) nanofibers with cylindrical (400±200 nm diameter) and ribbon-like (80±10 nm thick ribbons with the width-to-thickness ratio of up to 50:1) shapes were prepared by annealing electrospun polyvinylpyrrolidone/ammonium metatungstate (PVP/AMT) fibers with 1:1 PVP/AMT weight ratio. The effects of annealing temperature (573–1173 K), time (1–12 h), and fibers׳ shape on their crystalline structure and stability were investigated. Only a polycrystalline monoclinic phase of WO3 was observed in all experiments, and the grain size increased with temperature and time from a few nanometers to ~1 μm. Results have shown that thermal stability of fibers depend on their shape and whether or not the fibers are clamped to the substrate. Loose fibers of both types were stable to at least 873 K. Ribbon-like structures degraded between 873 K and 973 K, whereas some fraction of cylindrical fibers survived annealing at 1173 K. When both types of fibers were clamped to silicon substrate, annealing above 873 K led to their partial disintegration with the formation of large needle and lamellar WO3 crystals.